1 sql. motivation in principle, we can just use relational algebra to query the tables 2

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SQL

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Motivation• In principle, we can just use relational algebra to

query the tables

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Example

• Find all bars that sell beers above $2.5

PROJECT bar (SELECT price>2.5 Sells)

Sells( bar, beer, price ) Joe’s Bud 2.50 Joe’s Miller 2.75 Sue’s Bud 2.50Joe’s Coors2.50

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Motivation• But this may be hard for ordinary programmers to

write– especially when the query becomes complex

• Better devise a query language that is more “English”, more understandable

• That would be SQL• A SQL query can be translated into a RA

expression (or tree)

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SQL Introduction

Standard language for querying and manipulating data

Structured Query Language

Many standards out there: SQL92, SQL2, SQL3, SQL99Vendors support various subsets of these, but all of what we’llbe talking about.

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Select-From-Where StatementsMeaning of Queries

Subqueries

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Select-From-Where Statements• The principal form of a query is:

SELECT desired attributes

FROM one or more tables

WHERE condition about tuples of

the tables

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Single-Relation Queries

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Our Running Example• Most of our SQL queries will be based on the following

database schema.– Underline indicates key attributes.

Beers(name, manf)Bars(name, addr, license)Drinkers(name, addr, phone)Likes(drinker, beer)Sells(bar, beer, price)Frequents(drinker, bar)

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ExampleSells( bar, beer, price ) Bars(name, addr )

Joe’s Bud 2.50 Joe’s Maple St.Joe’s Miller 2.75 Sue’s River Rd.Sue’s Bud 2.50Sue’s Coors3.00

name manfBud Anheuser-Busch... ...

Beers

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Example

• Using Beers(name, manf), what beers are made by Anheuser-Busch?

SELECT nameFROM BeersWHERE manf = ‘Anheuser-

Busch’;

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Result of Query

name

‘Bud’

‘Bud Lite’

‘Michelob’

The answer is a relation with a single attribute,name, and tuples with the name of each beerby Anheuser-Busch, such as Bud.

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Meaning of Single-Relation Query

• Begin with the relation in the FROM clause.• Apply the selection indicated by the WHERE

clause.• Apply the projection indicated by the SELECT

clause.

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Operational Semantics

• To implement this algorithm think of a tuple variable ranging over each tuple of the relation mentioned in FROM.

• Check if the “current” tuple satisfies the WHERE clause.

• If so, compute the attributes or expressions of the SELECT clause using the components of this tuple.

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* In SELECT clauses

• When there is one relation in the FROM clause, * in the SELECT clause stands for “all attributes of this relation.”

• Example using Beers(name, manf):

SELECT *FROM BeersWHERE manf = ‘Anheuser-

Busch’;

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Result of Query:

name manf

‘Bud’ ‘Anheuser-Busch’

‘Bud Lite’ ‘Anheuser-Busch’

‘Michelob’ ‘Anheuser-Busch’

Now, the result has each of the attributesof Beers.

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Another Example

Company(sticker, name, country, stockPrice)

Find all US companies whose stock is > 50:

Output schema: R(sticker, name, country, stockPrice)

SELECT *FROM CompanyWHERE country=“USA” AND stockPrice > 50

SELECT *FROM CompanyWHERE country=“USA” AND stockPrice > 50

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Renaming Attributes

• If you want the result to have different attribute names, use “AS <new name>” to rename an attribute.

• Example based on Beers(name, manf):

SELECT name AS beer, manfFROM BeersWHERE manf = ‘Anheuser-

Busch’

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Result of Query:

beer manf

‘Bud’ ‘Anheuser-Busch’

‘Bud Lite’ ‘Anheuser-Busch’

‘Michelob’ ‘Anheuser-Busch’

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Expressions in SELECT Clauses

• Any expression that makes sense can appear as an element of a SELECT clause.

• Example: from Sells(bar, beer, price):

SELECT bar, beer,price * 120 AS

priceInYenFROM Sells;

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ExampleSells( bar, beer, price ) Bars(name, addr )

Joe’s Bud 2.50 Joe’s Maple St.Joe’s Miller 2.75 Sue’s River Rd.Sue’s Bud 2.50Sue’s Coors3.00

name manfBud Anheuser-Busch... ...

Beers

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Result of Query

bar beer priceInYen

Joe’s Bud 300

Joe’s Miller 330

… … …

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Another Example: Constant Expressions

• From Likes(drinker, beer):

SELECT drinker,‘likes Bud’ AS

whoLikesBudFROM LikesWHERE beer = ‘Bud’;

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Result of Query

drinker whoLikesBud

‘Sally’ ‘likes Bud’

‘Fred’ ‘likes Bud’

… …

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Complex Conditions in WHERE Clause

• From Sells(bar, beer, price), find the price Joe’s Bar charges for Bud:

SELECT priceFROM SellsWHERE bar = ‘Joe’’s Bar’ AND

beer = ‘Bud’;

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Selections

What you can use in WHERE: attribute names of the relation(s) used in the FROM. comparison operators: =, <>, <, >, <=, >= apply arithmetic operations: stockprice*2 operations on strings (e.g., “||” for concatenation). Lexicographic order on strings. Pattern matching: s LIKE p Special stuff for comparing dates and times.

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Important Points

• Two single quotes inside a string represent the single-quote (apostrophe).

• Conditions in the WHERE clause can use AND, OR, NOT, and parentheses in the usual way boolean conditions are built.

• SQL is case-insensitive. In general, upper and lower case characters are the same, except inside quoted strings.

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Patterns

• WHERE clauses can have conditions in which a string is compared with a pattern, to see if it matches.

• General form: <Attribute> LIKE <pattern> or <Attribute> NOT LIKE <pattern>

• Pattern is a quoted string with % = “any string”; _ = “any character.”

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Example

• From Drinkers(name, addr, phone) find the drinkers with exchange 555:

SELECT nameFROM DrinkersWHERE phone LIKE ‘%555-_ _ _ _’;

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The LIKE operator• s LIKE p: pattern matching on strings• p may contain two special symbols:

– % = any sequence of characters– _ = any single character

Company(sticker, name, address, country, stockPrice)Find all US companies whose address contains “Mountain”:

SELECT *FROM CompanyWHERE country=“USA” AND address LIKE “%Mountain%”

SELECT *FROM CompanyWHERE country=“USA” AND address LIKE “%Mountain%”

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Motivating Example for Next Few Slides• From the following Sells relation:

bar beer price

.... .... ...

SELECT bar

FROM Sells

WHERE price < 2.00 OR price >= 2.00;

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Null Values

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NULL Values

• Tuples in SQL relations can have NULL as a value for one or more components.

• Meaning depends on context. Two common cases:– Missing value : e.g., we know Joe’s Bar has

some address, but we don’t know what it is.– Inapplicable : e.g., the value of attribute spouse

for an unmarried person.

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Cheat Sheet for Working with NULL• SQL will evaluate to TRUE only if it knows for

*sure*• SQL will return only cases for which it evaluates

to TRUE• Example: (price < 2.00)

– What happens if price = NULL? – Intuitively, price could be less than 2.00, or greater or

equal to 2.00 doesn’t know for *sure*.

• Example: (price >= 2.00)• Example: (price < 2.00) OR (price >= 2.00)

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Example• From the following Sells relation:

bar beer price

Joe’s Bar Bud NULL

SELECT bar

FROM Sells


UNKNOWN UNKNOWN

UNKNOWN

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Comparing NULL’s to Values

• The logic of conditions in SQL is really 3-valued logic: TRUE, FALSE, UNKNOWN.– (price < 2.00) OR (price >= 3.00)

• When any value is compared with NULL, the truth value is UNKNOWN.

• But a query only produces a tuple in the answer if its truth value for the WHERE clause is TRUE (not FALSE or UNKNOWN).

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Three-Valued Logic

• To understand how AND, OR, and NOT work in 3-valued logic, think of TRUE = 1, FALSE = 0, and UNKNOWN = ½.

• AND = MIN; OR = MAX, NOT(x) = 1-x.• Example:• (age > 20) AND ((age < 10) OR NOT (price

< 5))

TRUE AND (FALSE OR NOT(UNKNOWN)) = MIN(1, MAX(0, (1 - ½ ))) =

MIN(1, MAX(0, ½ ) = MIN(1, ½ ) = ½.

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Surprising Example• From the following Sells relation:

bar beer price

Joe’s Bar Bud NULL

SELECT bar

FROM Sells


UNKNOWN UNKNOWN

UNKNOWN

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Reason: 2-Valued Laws != 3-Valued Laws

• Some common laws, like the commutativity of AND, hold in 3-valued logic.

• But others do not; example: the “law of excluded middle,” p OR NOT p = TRUE.– When p = UNKNOWN, the left side is MAX( ½, (1 –

½ )) = ½ != 1.

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Null Values• If x=Null then 4*(3-x)/7 is still NULL

• If x=Null then x=“Joe” is UNKNOWN• Three boolean values:

– FALSE = 0– UNKNOWN = 0.5– TRUE = 1

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Null Value Logic

• C1 AND C2 = min(C1, C2)• C1 OR C2 = max(C1, C2)• NOT C1 = 1 – C1

SELECT *FROM PersonWHERE (age < 25) AND (height > 6 OR weight > 190)

Semantics of SQL: include only tuples that yield TRUE

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Null Values

Unexpected behavior:

SELECT *

FROM Person

WHERE age < 25 OR age >= 25

Some Persons are not included !

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Testing for Null

Can test for NULL explicitly:– x IS NULL– x IS NOT NULL

SELECT *FROM PersonWHERE age < 25 OR age >= 25 OR age IS NULL

Now it includes all Persons

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Multi-Relation Queries

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Multirelation Queries• Interesting queries often combine data from more

than one relation.• We can address several relations in one query by

listing them all in the FROM clause.• Distinguish attributes of the same name by

“<relation>.<attribute>”

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Example

• Using relations Likes(drinker, beer) and Frequents(drinker, bar), find the beers liked by at least one person who frequents Joe’s Bar.

SELECT beerFROM Likes, FrequentsWHERE bar = ‘Joe’’s Bar’ AND

Frequents.drinker = Likes.drinker;

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Another Example

Product (pname, price, category, maker)Purchase (buyer, seller, store, product)Company (cname, stockPrice, country)Person(pname, phoneNumber, city)

Find names of people living in Champaign that bought gizmo products, and the names of the stores they bought from

SELECT Person.pname, storeFROM Person, PurchaseWHERE Person.pname=buyer AND city=“Champaign” AND product=“gizmo”

SELECT Person.pname, storeFROM Person, PurchaseWHERE Person.pname=buyer AND city=“Champaign” AND product=“gizmo”

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Disambiguating Attributes

Product (name, price, category, maker)Purchase (buyer, seller, store, product)Person(name, phoneNumber, city)

Find names of people buying products of category “telephony”:

SELECT Person.nameFROM Person, Purchase, ProductWHERE Person.name=buyer AND Purchase.product=Product.name AND Product.category=“telephony”

SELECT Person.nameFROM Person, Purchase, ProductWHERE Person.name=buyer AND Purchase.product=Product.name AND Product.category=“telephony”

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Formal Semantics• Almost the same as for single-relation queries:

1. Start with the Cartesian product of all the relations in the FROM clause.

2. Apply the selection condition from the WHERE clause.

3. Project onto the list of attributes and expressions in the SELECT clause.

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Operational Semantics• Imagine one tuple-variable for each relation in

the FROM clause.– These tuple-variables visit each combination of

tuples, one from each relation.

• If the tuple-variables are pointing to tuples that satisfy the WHERE clause, send these tuples to the SELECT clause.

drinker bar drinker beer

tv1 tv2Sally Bud

Sally Joe’s

to outputcheck theseare equal

checkfor Joe

LikesFrequents

SELECT beerFROM Frequents, LikesWHERE bar = ‘Joe’’s Bar’ AND


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Can always do tuple variables

SELECT y.beerFROM Frequents AS x, Likes AS yWHERE x.bar = ‘Joe’’s Bar’ AND

x.drinker = y.drinker;

SELECT beerFROM Frequents, LikesWHERE bar = ‘Joe’’s Bar’ AND


SELECT y.beerFROM Frequents x, Likes yWHERE x.bar = ‘Joe’’s Bar’ AND

x.drinker = y.drinker;

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Explicit Tuple-Variables

• Sometimes, a query needs to use two copies of the same relation.

• Distinguish copies by following the relation name by the name of a tuple-variable, in the FROM clause.

• It’s always an option to rename relations this way, even when not essential.

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Example• From Beers(name, manf), find all pairs of

beers by the same manufacturer.– Do not produce pairs like (Bud, Bud).– Produce pairs in alphabetic order, e.g. (Bud,

Miller), not (Miller, Bud).

name manf

Bud X

Miller X

Spotted cow Y

Spotted badger Z

Spotted deer Y

name1 name2

Bud Miller

Spotted cow Spotted deer

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SELECT b1.name as name1, b2.name as name2FROM Beers b1, Beers b2WHERE b1.manf = b2.manf AND

b1.name < b2.name;

name manf

Bud X

Miller X

Spotted cow Y

Spotted badger Z

Spotted deer Y

name manf

Bud X

Miller X

Spotted cow Y

Spotted badger Z

Spotted deer Y

name1 name2

Bud Miller

Spotted cow Spotted deer

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Tuple Variables

SELECT product1.maker, product2.makerFROM Product AS product1, Product AS product2WHERE product1.category=product2.category AND product1.maker <> product2.maker

SELECT product1.maker, product2.makerFROM Product AS product1, Product AS product2WHERE product1.category=product2.category AND product1.maker <> product2.maker

Find pairs of companies making products in the same category

Product ( name, price, category, maker)

A 3 X M

B 4 X M

C 3 X P

D 6 Y M

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Tuple VariablesTuple variables introduced automatically by the system: Product ( name, price, category, maker)

Becomes:

Doesn’t work when Product occurs more than once:In that case the user needs to define variables explicitly.

SELECT name FROM Product WHERE price > 100

SELECT name FROM Product WHERE price > 100

SELECT Product.name FROM Product AS Product WHERE Product.price > 100

SELECT Product.name FROM Product AS Product WHERE Product.price > 100

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Meaning (Semantics) of SQL Queries

SELECT a1, a2, …, akFROM R1 AS x1, R2 AS x2, …, Rn AS xnWHERE Conditions

1. Nested loops:

Answer = {}for x1 in R1 do for x2 in R2 do ….. for xn in Rn do if Conditions then Answer = Answer U {(a1,…,ak)return Answer

Answer = {}for x1 in R1 do for x2 in R2 do ….. for xn in Rn do if Conditions then Answer = Answer U {(a1,…,ak)return Answer

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2. Parallel assignment

Doesn’t impose any order !

Answer = {}for all assignments x1 in R1, …, xn in Rn do if Conditions then Answer = Answer U {(a1,…,ak)}return Answer

Answer = {}for all assignments x1 in R1, …, xn in Rn do if Conditions then Answer = Answer U {(a1,…,ak)}return Answer

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3. Translation to Relational algebra:

Pa1,…,ak ( s Conditions (R1 x R2 x … x Rn))

Select-From-Where queries are precisely Select-Project-Join

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Exercises

Product ( pname, price, category, maker)Purchase (buyer, seller, store, product)Company (cname, stock price, country)Person( per-name, phone number, city)

Ex #1: Find people who bought products of category “telephony”

Select buyerFrom Product, PurchaseWhere (product = pname) and (category = telephony)

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Exercises


Ex #2: Find names of people who bought American products

SelectFrom Purchase P, Company C, Product ProWhere (P.product = Pro.pname) and (Pro.maker = C.cname)

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Exercises


Ex #3: Find names of people who bought American products and they live in Madison

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Exercises Product ( pname, price, category, maker)Purchase (buyer, seller, store, product)Company (cname, stock price, country)Person( per-name, phone number, city)

Ex #4: Find the names of people who bought stuff from Joe or bought products from a company whose stock prices is more than $50.

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Subqueries

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Subqueries• A parenthesized SELECT-FROM-WHERE

statement (subquery) can be used as a value in a number of places, including FROM and WHERE clauses.

• Example: in place of a relation in the FROM clause, we can place another query, and then query its result.– Better use a tuple-variable to name tuples of the

result.

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Example

Select p1.maker, p2.maker

From Product p1, Product p2

Where (p1.price = p2.price) and p1.category = iphone) and (p2.category = iphone) and (p1.maker <> p2.maker)


Ex #5: Find pair of companies that sell iphone product at the same price

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Example

Select p1.maker, p2.maker

From (Select * From Product Where category = iphone) as p1

(Select * From Product

Where category = iphone) as p2

Where (p1.price = p2.price) and (p1.maker <> p2.maker)


Ex #5: Find pair of companies that sell iphone product at the same price

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Subqueries That Return One Tuple

• If a subquery is guaranteed to produce one tuple, then the subquery can be used as a value.– Usually, the tuple has one component.– Also typically, a single tuple is guaranteed by

keyness of attributes.– A run-time error occurs if there is no tuple or

more than one tuple.

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Example

• From Sells(bar, beer, price), find the bars that serve Miller for the same price Joe charges for Bud.

• Two queries would surely work:1. Find the price Joe charges for Bud.

2. Find the bars that serve Miller at that price.

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Query + Subquery Solution

SELECT bar

FROM Sells

WHERE beer = ‘Miller’ AND

price = (SELECT price

FROM Sells

WHERE bar = ‘Joe’’s Bar’

AND beer = ‘Bud’);The price atwhich Joesells Bud

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The IN Operator• <tuple> IN <relation> is true if and only if the

tuple is a member of the relation.– <tuple> NOT IN <relation> means the opposite.

• IN-expressions can appear in WHERE clauses.• The <relation> is often a subquery.

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Example

• From Beers(name, manf) and Likes(drinker, beer), find the name and manufacturer of each beer that Fred likes.

SELECT *

FROM Beers

WHERE name IN (SELECT beer

FROM Likes

WHERE drinker = ‘Fred’);The set ofbeers Fredlikes

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The Exists Operator• EXISTS( <relation> ) is true if and only if the

<relation> is not empty.• Being a boolean-valued operator, EXISTS can

appear in WHERE clauses.• Example: From Beers(name, manf), find those

beers that are the unique beer by their manufacturer.

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Example Query with EXISTS

SELECT name

FROM Beers b1

WHERE NOT EXISTS(

SELECT *

FROM Beers

WHERE manf = b1.manf AND

name <> b1.name);

Set ofbeerswith thesamemanf asb1, butnot thesamebeer

Notice scope rule: manf refersto closest nested FROM witha relation having that attribute.

Notice theSQL “notequals”operator

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The Operator ANY

• x = ANY( <relation> ) is a boolean condition meaning that x equals at least one tuple in the relation.

• Similarly, = can be replaced by any of the comparison operators.

• Example: x >= ANY( <relation> ) means x is not smaller than all tuples in the relation.– Note tuples must have one component only.

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The Operator ALL

• Similarly, x <> ALL( <relation> ) is true if and only if for every tuple t in the relation, x is not equal to t.– That is, x is not a member of the relation.

• The <> can be replaced by any comparison operator.

• Example: x >= ALL( <relation> ) means there is no tuple larger than x in the relation.

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Example• From Sells(bar, beer, price), find the beer(s) sold

for the highest price.

SELECT beer

FROM Sells

WHERE price >= ALL(

SELECT price

FROM Sells);

price from the outerSells must not beless than any price.

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More SQL

Relations as BagsGrouping and AggregationDatabase Modification

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Relational Algebra: Operations on Bags (and why we care)

• Union: {a,b,b,c} U {a,b,b,b,e,f,f} = {a,a,b,b,b,b,b,c,e,f,f}– add the number of occurrences

• Difference: {a,b,b,b,c,c} – {b,c,c,c,d} = {a,b,b,d}– subtract the number of occurrences

• Intersection: {a,b,b,b,c,c} {b,b,c,c,c,c,d} = {b,b,c,c}– minimum of the two numbers of occurrences

• Selection: preserve the number of occurrences• Projection: preserve the number of occurrences (no duplicate

elimination)• Cartesian product, join: no duplicate elimination

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Bag Semantics for SFW Queries• The SELECT-FROM-WHERE statement uses

bag semantics– Selection: preserve the number of occurrences– Projection: preserve the number of occurrences (no

duplicate elimination)– Cartesian product, join: no duplicate elimination

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Union, Intersection, and Difference

• Union, intersection, and difference of relations are expressed by the following forms, each involving subqueries:– ( subquery ) UNION ( subquery )– ( subquery ) INTERSECT ( subquery )– ( subquery ) EXCEPT ( subquery )

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Example• From relations Likes(drinker, beer), Sells(bar,

beer, price) and Frequents(drinker, bar), find the drinkers and beers such that:

1. The drinker likes the beer, and

2. The drinker frequents at least one bar that sells the beer.

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Solution

(SELECT * FROM Likes)

INTERSECT

(SELECT drinker, beer

FROM Sells, Frequents

WHERE Frequents.bar = Sells.bar

);

The drinker frequentsa bar that sells thebeer.

Likes(drinker, beer)

Sells(bar, beer, price)

Frequents(drinker, bar)

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Bag Semantics for Set Operations in SQL• Although the SELECT-FROM-WHERE

statement uses bag semantics, the default for union, intersection, and difference is set semantics.– That is, duplicates are eliminated as the operation is

applied.

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Motivation: Efficiency

• When doing projection in relational algebra, it is easier to avoid eliminating duplicates.– Just work tuple-at-a-time.

• When doing intersection or difference, it is most efficient to sort the relations first.– At that point you may as well eliminate the

duplicates anyway.

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Example

X

name

a

b

a

c

d

Y

name

c

c

a

a

b

• To compute X intersect Y– first sort X and Y– then compute intersection– doing duplicate elimination along the

way

– X = {a, a, b, c, d}– Y = {a, a, b, c, c}– result = {a, b, c}

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Controlling Duplicate Elimination

• Force the result to be a set by SELECT DISTINCT . . .

• Force the result to be a bag (i.e., don’t eliminate duplicates) by ALL, as in . . . UNION ALL . . .

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Example: DISTINCT• From Sells(bar, beer, price), find all the different

prices charged for beers:

SELECT DISTINCT priceFROM Sells;

• Notice that without DISTINCT, each price would be listed as many times as there were bar/beer pairs at that price.

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Example: ALL

• Using relations Frequents(drinker, bar) and Likes(drinker, beer):

(SELECT drinker FROM Frequents)

EXCEPT ALL(SELECT drinker FROM Likes);

• Lists drinkers who frequent more bars than they like beers, and does so as many times as the difference of those counts.

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Join Expressions• SQL provides a number of expression forms that

act like varieties of join in relational algebra.– But using bag semantics, not set semantics.

• These expressions can be stand-alone queries or used in place of relations in a FROM clause.

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Products and Natural Joins

• Natural join is obtained by:

R NATURAL JOIN S;• Product is obtained by:

R CROSS JOIN S;• Example:

Likes NATURAL JOIN Serves;• Relations can be parenthesized subexpressions, as

well.

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Theta Join

• R JOIN S ON <condition> is a theta-join, using <condition> for selection.

• Example: using Drinkers(name, addr) and Frequents(drinker, bar):

Drinkers JOIN Frequents ONname = drinker;

gives us all (d, a, d, b) quadruples such that drinker d lives at address a and frequents bar b.

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Motivation for OuterjoinsExplicit joins in SQL:

Product(name, category) Purchase(prodName, store)

SELECT Product.name, Purchase.store

FROM Product JOIN Purchase ON

Product.name = Purchase.prodName

Same as:


FROM Product, Purchase

WHERE Product.name = Purchase.prodName

But Products that never sold will be lost !

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Null Values and Outerjoins

Left outer joins in SQL:Product(name, category)

Purchase(prodName, store)


FROM Product LEFT OUTER JOIN Purchase ON

Product.name = Purchase.prodName

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Name Category

Gizmo gadget

Camera Photo

OneClick Photo

ProdName Store

Gizmo Wiz

Camera Ritz

Camera Wiz

Name Store

Gizmo Wiz

Camera Ritz

Camera Wiz

OneClick -

Product Purchase

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Outer Joins

• Left outer join:– Include the left tuple even if there’s no match

• Right outer join:– Include the right tuple even if there’s no match

• Full outer join:– Include the both left and right tuples even if there’s no

match

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Outerjoins

• R OUTER JOIN S is the core of an outerjoin expression. It is modified by:

1. Optional NATURAL in front of OUTER.2. Optional ON <condition> after JOIN.3. Optional LEFT, RIGHT, or FULL before

OUTER. LEFT = pad dangling tuples of R only. RIGHT = pad dangling tuples of S only. FULL = pad both; this choice is the default.

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Aggregations• SUM, AVG, COUNT, MIN, and MAX can be

applied to a column in a SELECT clause to produce that aggregation on the column.

• Also, COUNT(*) counts the number of tuples.

100

Example: Aggregation• From Sells(bar, beer, price), find the average

price of Bud:

SELECT AVG(price)FROM SellsWHERE beer = ‘Bud’;

101

Eliminating Duplicates in an Aggregation

• DISTINCT inside an aggregation causes duplicates to be eliminated before the aggregation.

• Example: find the number of different prices charged for Bud:

SELECT COUNT(DISTINCT price)FROM SellsWHERE beer = ‘Bud’;

102

NULL’s Ignored in Aggregation• NULL never contributes to a sum, average, or

count, and can never be the minimum or maximum of a column.

• But if there are no non-NULL values in a column, then the result of the aggregation is NULL.

103

Example: Effect of NULL’s

SELECT count(*)

FROM Sells

WHERE beer = ‘Bud’;

SELECT count(price)

FROM Sells

WHERE beer = ‘Bud’;

The number of barsthat sell Bud.

The number of barsthat sell Bud at aknown price.

104

Grouping• We may follow a SELECT-FROM-WHERE

expression by GROUP BY and a list of attributes.• The relation that results from the SELECT-

FROM-WHERE is grouped according to the values of all those attributes, and any aggregation is applied only within each group.

105

Example: Grouping• From Sells(bar, beer, price), find the average

price for each beer:

SELECT beer, AVG(price)FROM SellsGROUP BY beer;

106

Example: Grouping

• From Sells(bar, beer, price) and Frequents(drinker, bar), find for each drinker the average price of Bud at the bars they frequent:

SELECT drinker, AVG(price)

FROM Frequents, Sells

WHERE beer = ‘Bud’ AND

Frequents.bar = Sells.bar

GROUP BY drinker;

Computedrinker-bar-price of Budtuples first,then groupby drinker.

107

Restriction on SELECT Lists With Aggregation

• If any aggregation is used, then each element of the SELECT list must be either:

1. Aggregated, or

2. An attribute on the GROUP BY list.

108

Illegal Query Example

• You might think you could find the bar that sells Bud the cheapest by:

SELECT bar, MIN(price)FROM SellsWHERE beer = ‘Bud’;

• But this query is illegal in SQL.– Why? Note bar is neither aggregated nor on the

GROUP BY list.

109

HAVING Clauses• HAVING <condition> may follow a GROUP BY

clause.• If so, the condition applies to each group, and

groups not satisfying the condition are eliminated.

110

Requirements on HAVING Conditions

• These conditions may refer to any relation or tuple-variable in the FROM clause.

• They may refer to attributes of those relations, as long as the attribute makes sense within a group; i.e., it is either:

1. A grouping attribute, or

2. Aggregated.

111

Example: HAVING• From Sells(bar, beer, price) and Beers(name,

manf), find the average price of those beers that are either served in at least three bars or are manufactured by Pete’s.

112

Solution

SELECT beer, AVG(price)

FROM Sells

GROUP BY beer

HAVING COUNT(bar) >= 3 OR

beer IN (SELECT name

FROM Beers

WHERE manf = ‘Pete’’s’);

Beers manu-factured byPete’s.

Beer groups with at least3 non-NULL bars and alsobeer groups where themanufacturer is Pete’s.

113

General form of Grouping and Aggregation

SELECT S

FROM R1,…,Rn

WHERE C1

GROUP BY a1,…,ak

HAVING C2

S = may contain attributes a1,…,ak and/or any aggregates but NO OTHER ATTRIBUTES

C1 = is any condition on the attributes in R1,…,Rn

C2 = is any condition on aggregate expressions

114

General form of Grouping and Aggregation

SELECT S

FROM R1,…,Rn

WHERE C1

GROUP BY a1,…,ak

HAVING C2

Evaluation steps:1. Compute the FROM-WHERE part, obtain a table with all attributes

in R1,…,Rn

2. Group by the attributes a1,…,ak 3. Compute the aggregates in C2 and keep only groups satisfying C24. Compute aggregates in S and return the result

115

Database Modification

116

Database Modifications• A modification command does not return a

result as a query does, but it changes the database in some way.

• There are three kinds of modifications:1. Insert a tuple or tuples.

2. Delete a tuple or tuples.

3. Update the value(s) of an existing tuple or tuples.

117

Insertion• To insert a single tuple:

INSERT INTO <relation>

VALUES ( <list of values> );• Example: add to Likes(drinker, beer) the fact that

Sally likes Bud.

INSERT INTO LikesVALUES(‘Sally’, ‘Bud’);

118

Specifying Attributes in INSERT

• We may add to the relation name a list of attributes.

• There are two reasons to do so:1. We forget the standard order of attributes for the

relation.

2. We don’t have values for all attributes, and we want the system to fill in missing components with NULL or a default value.

119

Example: Specifying Attributes

• Another way to add the fact that Sally likes Bud to Likes(drinker, beer):

INSERT INTO Likes(beer, drinker)VALUES(‘Bud’, ‘Sally’);

120

Inserting Many Tuples• We may insert the entire result of a query into a

relation, using the form:

INSERT INTO <relation>

( <subquery> );

121

Example: Insert a Subquery• Using Frequents(drinker, bar), enter into the new

relation PotBuddies(name) all of Sally’s “potential buddies,” i.e., those drinkers who frequent at least one bar that Sally also frequents.

122

Solution

INSERT INTO PotBuddies

(SELECT d2.drinker

FROM Frequents d1, Frequents d2

WHERE d1.drinker = ‘Sally’ AND

d2.drinker <> ‘Sally’ AND

d1.bar = d2.bar

);

Pairs of Drinkertuples where thefirst is for Sally,the second is forsomeone else,and the bars arethe same.

The otherdrinker

123

Deletion• To delete tuples satisfying a condition from some

relation:

DELETE FROM <relation>

WHERE <condition>;

124

Example: Deletion• Delete from Likes(drinker, beer) the fact that

Sally likes Bud:

DELETE FROM LikesWHERE drinker = ‘Sally’ AND

beer = ‘Bud’;

125

Example: Delete all Tuples• Make the relation Likes empty:

DELETE FROM Likes;

• Note no WHERE clause needed.

126

Example: Delete Many Tuples• Delete from Beers(name, manf) all beers for

which there is another beer by the same manufacturer.

DELETE FROM Beers bWHERE EXISTS (

SELECT name FROM BeersWHERE manf = b.manf AND

name <> b.name);

Beers with the samemanufacturer anda different namefrom the name ofthe beer representedby tuple b.

127

Semantics of Deletion -- 1

• Suppose Anheuser-Busch makes only Bud and Bud Lite.

• Suppose we come to the tuple b for Bud first.

• The subquery is nonempty, because of the Bud Lite tuple, so we delete Bud.

• Now, When b is the tuple for Bud Lite, do we delete that tuple too?

128

Semantics of Deletion -- 2• The answer is that we do delete Bud Lite as

well.• The reason is that deletion proceeds in two

stages:1. Mark all tuples for which the WHERE condition is

satisfied in the original relation.

2. Delete the marked tuples.

129

Updates• To change certain attributes in certain tuples of a

relation:

UPDATE <relation>

SET <list of attribute assignments>

WHERE <condition on tuples>;

130

Example: Update• Change drinker Fred’s phone number to 555-

1212:

UPDATE DrinkersSET phone = ‘555-1212’WHERE name = ‘Fred’;

131

Example: Update Several Tuples

• Make $4 the maximum price for beer:

UPDATE SellsSET price = 4.00WHERE price > 4.00;

132

Defining a Database Schema

Views

133

Defining a Database Schema• A database schema comprises declarations for the

relations (“tables”) of the database.• Many other kinds of elements may also appear in

the database schema, including views, indexes, and triggers, which we’ll introduce later.

134

Declaring a Relation• Simplest form is:

CREATE TABLE <name> (

<list of elements>

);• And you may remove a relation from the

database schema by:

DROP TABLE <name>;

137

Elements of Table Declarations

• The principal element is a pair consisting of an attribute and a type.

• The most common types are:– INT or INTEGER (synonyms).– REAL or FLOAT (synonyms).– CHAR(n ) = fixed-length string of n

characters.– VARCHAR(n ) = variable-length string of up to

n characters.

138

Example: Create Table

CREATE TABLE Sells (bar CHAR(20),beer VARCHAR(20),price REAL

);

139

Dates and Times

• DATE and TIME are types in SQL.• The form of a date value is:

DATE ‘yyyy-mm-dd’– Example: DATE ‘2002-09-30’ for Sept. 30,

2002.

140

Times as Values

• The form of a time value is:

TIME ‘hh:mm:ss’

with an optional decimal point and fractions of a second following.– Example: TIME ’15:30:02.5’ = two and a half

seconds after 3:30PM.

141

Declaring Keys• An attribute or list of attributes may be declared

PRIMARY KEY or UNIQUE.• These each say the attribute(s) so declared

functionally determine all the attributes of the relation schema.

• There are a few distinctions to be mentioned later.

142

Declaring Single-Attribute Keys

• Place PRIMARY KEY or UNIQUE after the type in the declaration of the attribute.

• Example:

CREATE TABLE Beers (name CHAR(20)

UNIQUE,manf CHAR(20)

);

143

Declaring Multiattribute Keys• A key declaration can also be another element in

the list of elements of a CREATE TABLE statement.

• This form is essential if the key consists of more than one attribute.– May be used even for one-attribute keys.

144

Example: Multiattribute Key

• The bar and beer together are the key for Sells:

CREATE TABLE Sells (bar CHAR(20),beer VARCHAR(20),price REAL,PRIMARY KEY (bar, beer)

);

145

PRIMARY KEY Versus UNIQUE• The SQL standard allows DBMS implementers to

make their own distinctions between PRIMARY KEY and UNIQUE.– Example: some DBMS might automatically create an

index (data structure to speed search) in response to PRIMARY KEY, but not UNIQUE.

146

Required Distinctions• However, standard SQL requires these

distinctions:1. There can be only one PRIMARY KEY for a

relation, but several UNIQUE attributes.

2. No attribute of a PRIMARY KEY can ever be NULL in any tuple. But attributes declared UNIQUE may have NULL’s, and there may be several tuples with NULL.

147

Other Declarations for Attributes

• Two other declarations we can make for an attribute are:

1. NOT NULL means that the value for this attribute may never be NULL.

2. DEFAULT <value> says that if there is no specific value known for this attribute’s component in some tuple, use the stated <value>.

148

Example: Default Values

CREATE TABLE Drinkers (name CHAR(30) PRIMARY KEY,addr CHAR(50)

DEFAULT ‘123 Sesame St.’,phone CHAR(16)

);

149

Effect of Defaults -- 1• Suppose we insert the fact that Sally is a drinker,

but we know neither her address nor her phone.• An INSERT with a partial list of attributes makes

the insertion possible:

INSERT INTO Drinkers(name)VALUES(‘Sally’);

150

Effect of Defaults -- 2• But what tuple appears in Drinkers?

name addr phone

‘Sally’ ‘123 Sesame St’ NULL

• If we had declared phone NOT NULL, this insertion would have been rejected.

151

Adding Attributes

• We may change a relation schema by adding a new attribute (“column”) by:

ALTER TABLE <name> ADD

<attribute declaration>;• Example:

ALTER TABLE Bars ADDphone CHAR(16)DEFAULT ‘unlisted’;

152

Deleting Attributes

• Remove an attribute from a relation schema by:

ALTER TABLE <name>

DROP <attribute>;• Example: we don’t really need the license

attribute for bars:

ALTER TABLE Bars DROP license;

153

Views

• A view is a “virtual table,” a relation that is defined in terms of the contents of other tables and views.

• Declare by:

CREATE VIEW <name> AS <query>;• In contrast, a relation whose value is really

stored in the database is called a base table.

154

Example: View Definition

• CanDrink(drinker, beer) is a view “containing” the drinker-beer pairs such that the drinker frequents at least one bar that serves the beer:

CREATE VIEW CanDrink ASSELECT drinker, beerFROM Frequents, SellsWHERE Frequents.bar =

Sells.bar;

156

Example: Accessing a View

• You may query a view as if it were a base table.– There is a limited ability to modify views if the

modification makes sense as a modification of the underlying base table.

• Example:

SELECT beer FROM CanDrinkWHERE drinker = ‘Sally’;

157

What Happens When a View Is Used?• The DBMS starts by interpreting the query as if

the view were a base table.– Typical DBMS turns the query into something like

relational algebra.

• The queries defining any views used by the query are also replaced by their algebraic equivalents, and “spliced into” the expression tree for the query.

158

Example: View Expansion

PROJbeer

SELECTdrinker=‘Sally’

CanDrink

PROJdrinker, beer

JOIN

Frequents Sells

159

Constraints & Triggers

Foreign Keys

Local and Global Constraints

Triggers

160

Constraints and Triggers

• A constraint is a relationship among data elements that the DBMS is required to enforce.– Example: key constraints.

• Triggers are only executed when a specified condition occurs, e.g., insertion of a tuple.– Easier to implement than many constraints.

161

Kinds of Constraints

• Keys.• Foreign-key, or referential-integrity.• Value-based constraints.

– Constrain values of a particular attribute.

• Tuple-based constraints.– Relationship among components.

• Assertions: any SQL boolean expression.

162

Foreign Keys

• Consider Relation Sells(bar, beer, price).• We might expect that a beer value is a real

beer --- something appearing in Beers.name .

• A constraint that requires a beer in Sells to be a beer in Beers is called a foreign -key constraint.

165

Where are the foreign keys in these?• Most of our SQL queries will be based on the following

database schema.– Underline indicates key attributes.

Beers(name, manf)Bars(name, addr, license)Drinkers(name, addr, phone)Likes(drinker, beer)Sells(bar, beer, price)Frequents(drinker, bar)

166

Expressing Foreign Keys

• Use the keyword REFERENCES, either:1. Within the declaration of an attribute, when only

one attribute is involved.

2. As an element of the schema, as:

FOREIGN KEY ( <list of attributes> )

REFERENCES <relation> ( <attributes> )

• Referenced attributes must be declared PRIMARY KEY or UNIQUE.

167

Example: With Attribute

CREATE TABLE Beers (nameCHAR(20) PRIMARY KEY,manfCHAR(20) );

CREATE TABLE Sells (bar CHAR(20),beerCHAR(20) REFERENCES Beers(name),price REAL );

168

Example: As Element

CREATE TABLE Beers (nameCHAR(20) PRIMARY KEY,manfCHAR(20) );

CREATE TABLE Sells (bar CHAR(20),beerCHAR(20),price REAL,FOREIGN KEY(beer) REFERENCES Beers(name));

169

Enforcing Foreign-Key Constraints

• If there is a foreign-key constraint from attributes of relation R to the primary key of relation S, two violations are possible:

1. An insert or update to R introduces values not found in S.

2. A deletion or update to S causes some tuples of R to “dangle.”

170

Actions Taken -- 1

• Suppose R = Sells, S = Beers.• An insert or update to Sells that introduces a

nonexistent beer must be rejected.• A deletion or update to Beers that removes a

beer value found in some tuples of Sells can be handled in three ways.

171

Actions Taken -- 2

• The three possible ways to handle beers that suddenly cease to exist are:

1. Default : Reject the modification.

2. Cascade : Make the same changes in Sells. Deleted beer: delete Sells tuple. Updated beer: change value in Sells.

3. Set NULL : Change the beer to NULL.

172

Example: Cascade• Suppose we delete the Bud tuple from Beers.

– Then delete all tuples from Sells that have beer = ’Bud’.

• Suppose we update the Bud tuple by changing ’Bud’ to ’Budweiser’.– Then change all Sells tuples with beer = ’Bud’ so that

beer = ’Budweiser’.

173

Example: Set NULL• Suppose we delete the Bud tuple from Beers.

– Change all tuples of Sells that have beer = ’Bud’ to have beer = NULL.

• Suppose we update the Bud tuple by changing ’Bud’ to ’Budweiser’.– Same change.

174

Choosing a Policy

• When we declare a foreign key, we may choose policies SET NULL or CASCADE independently for deletions and updates.

• Follow the foreign-key declaration by:

ON [UPDATE, DELETE][SET NULL CASCADE]

• Two such clauses may be used.• Otherwise, the default (reject) is used.

175

Example

CREATE TABLE Sells (bar CHAR(20),beer CHAR(20),price REAL,FOREIGN KEY(beer)

REFERENCES Beers(name)ON DELETE SET NULLON UPDATE CASCADE );

176

Attribute-Based Checks• Put a constraint on the value of a particular

attribute.• CHECK( <condition> ) must be added to the

declaration for the attribute.• The condition may use the name of the attribute,

but any other relation or attribute name must be in a subquery.

177

Example

CREATE TABLE Sells (bar CHAR(20),beer CHAR(20) CHECK ( beer IN

(SELECT name FROM Beers)),price REAL CHECK ( price <= 5.00 )

);

178

Timing of Checks

• An attribute-based check is checked only when a value for that attribute is inserted or updated.– Example: CHECK (price <= 5.00) checks every

new price and rejects it if it is more than $5.– Example: CHECK (beer IN (SELECT name

FROM Beers)) not checked if a beer is deleted from Beers (unlike foreign-keys).

179

Tuple-Based Checks

• CHECK ( <condition> ) may be added as another element of a schema definition.

• The condition may refer to any attribute of the relation, but any other attributes or relations require a subquery.

• Checked on insert or update only.

180

Example: Tuple-Based Check

• Only Joe’s Bar can sell beer for more than $5:

CREATE TABLE Sells (bar CHAR(20),beer CHAR(20),price REAL,CHECK (bar = ’Joe’’s Bar’ OR

price <= 5.00));

181

Assertions• These are database-schema elements, like

relations or views.• Defined by:

CREATE ASSERTION <name>

CHECK ( <condition> );• Condition may refer to any relation or attribute in

the database schema.

182

Example: Assertion

• In Sells(bar, beer, price), no bar may charge an average of more than $5.

CREATE ASSERTION NoRipoffBars CHECK (NOT EXISTS (

SELECT bar FROM SellsGROUP BY barHAVING 5.00 < AVG(price)

));

Bars with anaverage priceabove $5

183

Example: Assertion• In Drinkers(name, addr, phone) and Bars(name, addr,

license), there cannot be more bars than drinkers.

CREATE ASSERTION FewBar CHECK ((SELECT COUNT(*) FROM Bars) <=(SELECT COUNT(*) FROM Drinkers)

);

184

Timing of Assertion Checks

• In principle, we must check every assertion after every modification to any relation of the database.

• A clever system can observe that only certain changes could cause a given assertion to be violated.– Example: No change to Beers can affect FewBar.

Neither can an insertion to Drinkers.

185

Triggers: Motivation

• Attribute- and tuple-based checks have limited capabilities.

• Assertions are sufficiently general for most constraint applications, but they are hard to implement efficiently.– The DBMS must have real intelligence to avoid

checking assertions that couldn’t possibly have been violated.

186

Triggers: Solution• A trigger allows the user to specify when the

check occurs.• Like an assertion, a trigger has a general-purpose

condition and also can perform any sequence of SQL database modifications.

187

Event-Condition-Action Rules

• Another name for “trigger” is ECA rule, or event-condition-action rule.

• Event : typically a type of database modification, e.g., “insert on Sells.”

• Condition : Any SQL boolean-valued expression.

• Action : Any SQL statements.

188

Example: A Trigger

• There are many details to learn about triggers.

• Here is an example to set the stage.• Instead of using a foreign-key constraint

and rejecting insertions into Sells(bar, beer, price) with unknown beers, a trigger can add that beer to Beers, with a NULL manufacturer.

189

Example: Trigger DefinitionCREATE TRIGGER BeerTrig

AFTER INSERT ON Sells

REFERENCING NEW ROW AS NewTuple

FOR EACH ROW

WHEN (NewTuple.beer NOT IN

(SELECT name FROM Beers))

INSERT INTO Beers(name)

VALUES(NewTuple.beer);

The event

The condition

The action

190

Options: CREATE TRIGGER• CREATE TRIGGER <name>• Option:

CREATE OR REPLACE TRIGGER <name>– Useful if there is a trigger with that name and you

want to modify the trigger.

191

Options: The Condition• AFTER can be BEFORE.

– Also, INSTEAD OF, if the relation is a view.• A great way to execute view modifications: have triggers

translate them to appropriate modifications on the base tables.

• INSERT can be DELETE or UPDATE.– And UPDATE can be UPDATE . . . ON a particular

attribute.

192

Options: FOR EACH ROW

• Triggers are either row-level or statement-level.

• FOR EACH ROW indicates row-level; its absence indicates statement-level.

• Row level triggers are executed once for each modified tuple.

• Statement-level triggers execute once for an SQL statement, regardless of how many tuples are modified.

193

Options: REFERENCING• INSERT statements imply a new tuple (for row-

level) or new set of tuples (for statement-level).• DELETE implies an old tuple or table.• UPDATE implies both.• Refer to these by

[NEW OLD][TUPLE TABLE] AS <name>

194

Options: The Condition

• Any boolean-valued condition is appropriate.

• It is evaluated before or after the triggering event, depending on whether BEFORE or AFTER is used in the event.

• Access the new/old tuple or set of tuples through the names declared in the REFERENCING clause.

195

Options: The Action• There can be more than one SQL statement in the

action.– Surround by BEGIN . . . END if there is more than

one.

• But queries make no sense in an action, so we are really limited to modifications.

196

Another Example• Using Sells(bar, beer, price) and a unary relation

RipoffBars(bar) created for the purpose, maintain a list of bars that raise the price of any beer by more than $1.

197

The Trigger

CREATE TRIGGER PriceTrigAFTER UPDATE OF price ON SellsREFERENCING

OLD ROW as oldNEW ROW as new

FOR EACH ROWWHEN(new.price > old.price + 1.00)INSERT INTO RipoffBars

VALUES(new.bar);

The event –only changesto prices

Updates let ustalk about oldand new tuples

We need to considereach price change

Condition:a raise inprice > $1

When the price changeis great enough, addthe bar to RipoffBars

198

Triggers on Views

• Generally, it is impossible to modify a view, because it doesn’t exist.

• But an INSTEAD OF trigger lets us interpret view modifications in a way that makes sense.

• Example: We’ll design a view Synergy that has (drinker, beer, bar) triples such that the bar serves the beer, the drinker frequents the bar and likes the beer.

199

Example: The View

CREATE VIEW Synergy AS

SELECT Likes.drinker, Likes.beer, Sells.bar

FROM Likes, Sells, Frequents

WHERE Likes.drinker = Frequents.drinker

AND Likes.beer = Sells.beer

AND Sells.bar = Frequents.bar;

Natural join of Likes,Sells, and Frequents

Pick one copy ofeach attribute

200

Interpreting a View Insertion• We cannot insert into Synergy --- it is a view.• But we can use an INSTEAD OF trigger to turn a

(drinker, beer, bar) triple into three insertions of projected pairs, one for each of Likes, Sells, and Frequents.– The Sells.price will have to be NULL.

201

The Trigger

CREATE TRIGGER ViewTrig

INSTEAD OF INSERT ON Synergy

REFERENCING NEW ROW AS n

FOR EACH ROW

BEGIN

INSERT INTO LIKES VALUES(n.drinker, n.beer);

INSERT INTO SELLS(bar, beer) VALUES(n.bar, n.beer);

INSERT INTO FREQUENTS VALUES(n.drinker, n.bar);

END;

1 sql. motivation in principle, we can just use relational algebra to query the tables 2

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