1

Introduction to SQL

Select-From-Where StatementsSubqueriesGrouping and Aggregation

2

Why SQL?

SQL is a very-high-level language. Say “what to do” rather than “how to do it.” Avoid a lot of data-manipulation details needed

in procedural languages like C++ or Java.

Database management system figures out “best” way to execute query. Called “query optimization.”

3

Select-From-Where Statements

SELECT desired attributes

FROM one or more tables

WHERE condition about tuples of the tables;

SELECT title, length

FROM movies

WHERE year =1994;

For example:

4

Our Running Example

All our SQL queries will be based on the following database schema:

Movie(title, year, length, inColor, studioName, producerC#)

StarsIn(movieTitle, movieYear, starName) MovieStar(name, address, gender, birthdate) MovieExec(name, address, cert#, netWorth) Studio(name, address, preaC#)

5

Example

Using Movie, what movies were produced by Disney Studios in 1990?

SELECT title, length

FROM Movie

WHERE studioName = ‘Disney’ AND year = 1990;

Notice SQL uses single-quotes for strings.SQL is case-insensitive, except inside strings.

6

Result of Query

title length

Pretty Women

…

119

7

Meaning of Single-Relation Query

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

clause. Apply the extended projection indicated by

the SELECT clause.

8

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.

9

* 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 Movie:

SELECT *

FROM Movie

WHERE studioName = ‘Disney’ AND year = 1990;

10

Result of Query:

title year length inColor studioName procucerC#

Pretty Women

…

1990 119 true Disney 999

11

Renaming Attributes

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

Example based on Movie: SELECT title As name, length As duration FROM Movie WHERE studioName = ‘Disney’ AND year = 1990;

12

Result of Query:

name Duration

Pretty Women

…

119

13

Expressions in SELECT Clauses

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

Example: from Movie:SELECT title As name, length*0.016667 As lengthInHours

FROM Movie;

14

Constant Expressions

From Movie :

SELECT title, length*0.016667 AS length, ‘hrs.’ AS inHours

FROM Movie

WHERE studioName = ‘Disney’ AND year = 1990;

15

Result of Query

title length inHours

Pretty Women

…

1.98334 hrs.

16

Complex Conditions in WHERE Clause

From Movie, find all the movies made after 1970 that are in black-and-white:

SELECT title

FROM Movie

WHERE year > 1970 AND NOT inColor;

17

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) or _ (any character)

18

Example

From Movie find all the movies have the title as “Star something”, and we remember that something has four letters.:

SELECT titleFROM MovieWHERE title LIKE ‘Star _ _ _ _’;

A ‘_’ just represent half Chinese characters

19

Escape characters in LIKE expressions

If the pattern we wish to use in a LIKE expression involves the characters % or _. We can follow the pattern by the keyword ESCAPE and choose a escape character.

For example:a LIKE ‘x%%x%’ ESCAPE ‘x’

20

Comparing dates and times

A date is represented by the keyword DATE followed by a quoted string of a special form. For example, DATE ‘1948-05-14’ follows the required form.

A time is represented similarly by the keyword TIME and a quoted string. For instance, TIME ‘ 15:00:02.5’

We can compare dates or times using the same comparison operators we use for numbers or strings.

21

Ordering the output

To get output in sorted order, we add to the select-from-where statement a clause:ORDER BY <list of attributes>

The order is by default ascending (ASC), but we can get the output highest-first by appending the keyword DESC.

22

Example

To get the movies listed by length, shortest first, and among movies of equal length, alphabetically, we can say:

SELECT *

FROM Movie

WHERE studioName = ‘Disney’ AND year = 1990

ORDER BY length, title;

23

练习

图书 ( 书号 , 书名 , 作者 , 出版社 , 单价） 查询“数据库”一书的书号和单价 查询所有图书的名称和单价，并按单价从大到小排序 查询单价在 20至 50元之间的图书信息 查询北京某出版社出版的图书信息 查询作者是张一，王二，刘三的书的信息 查询所有图书的书号，书名和半价信息 查询缺少出版社信息的图书的书号和书名

24

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>”

25

Example

Using relations Movie and MovieExec, find the name of the producer of Star War.

SELECT name

FROM Movie, MovieExec

WHERE title = ‘Star War’ AND producerC# = cert#;

26

Formal Semantics

Almost the same as for single-relation queries:

1. Start with the 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.

27

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.

28

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.

29

Example

From MovieStar, find all pairs of stars who share an address.

SELECT Star1.name, Star2.name

FROM MovieStar AS Star1, MovieStar AS Star2

WHERE Star1.address = Star2.address AND

Star1.name <> Star2.name;

30

练习

读者 ( 读者编号，姓名，电话 )图书 ( 书号 , 书名 , 作者 , 出版社 , 单价）借阅 ( 书号，读者编号，借阅日期 )

查询借阅过书号为‘ J0004’图书的读者姓名 查询王明所借阅的所有图书的书名和借阅日期

31

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.

32

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. A run-time error occurs if there is no tuple or

more than one tuple.

33

Example

From Movie, MovieExec, find the name of the producer of Star War.

Two queries would surely work:1. Find the certificate number for the producer of

Star War.

2. Find the name of the person with this certificate.

34

Query + Subquery Solution

SELECT name

FROM MovieExec

WHERE cert# =

(SELECT producerC#

FROM Movie

WHERE title = ‘Star Wars’);

35

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.

36

Example From Movie, MovieExec and StarsIn, find all the producers of

movies in which Harrison Ford stars. SELECT name FROM MovieExec WHERE cert# IN

(SELECT producerC# FROM Movie WHERE (title, year) IN

(SELECT movieTitle, movieYear FROM StarsIn WHERE starName = ‘Harrison Ford’));

The nested query can be written as a single select-from-where expression?

37

The Exists Operator

EXISTS( <relation> ) is true if and only if the <relation> is not empty.

Example: From Beers(name, manf) , find those beers that are the unique beer by their manufacturer.

38

Example Query with EXISTS

SELECT name

FROM Beers b1

WHERE NOT EXISTS(

SELECT *

FROM Beers

WHERE manf = b1.manf AND name <> b1.name);

39

The Operator ANY

x = ANY( <relation> ) is a boolean condition true if 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 the smallest tuple in the relation. Note tuples must have one component only.

40

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.

41

Example

From Movie, find the titles that have been used for two or more movies.

SELECT title

FROM Movie As old

WHERE year < ANY(

SELECT year

FROM Movie

WHERE title = old.title);

42

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 )

43

Example Using MovieStar and MovieExec, suppose we wanted the

names and addresses of all female movie stars who are also movie executives with a net worth over $10,000,000.

(SELECT name, addressFROM MovieStarWHERE gender = ‘F’) INTERSECT (SELECT name, address FROM MovieExec WHERE netWorth > 10000000);

44

Controlling Duplicate Elimination

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

45

Example: DISTINCT

From Movie, MovieExec, StarIn, find all the producers of movies in which Harrison Ford stars :

SELECT DISTINCT nameFROM Movie, MovieExec, StarsIn

WHERE cert# = producerC# AND tile = movieTitle AND

year = moiveYear AND starName = ‘Harrison Ford’;

Notice that without DISTINCT, each name would be listed many times.

46

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.

47

Example: Aggregation

From MovieExec, find the average net worth of all movie executives:

SELECT AVG(netWorth)

FROM MovieExec;

48

Eliminating Duplicates in an Aggregation

Use DISTINCT inside an aggregation. Example: find the number of different name

in MovieExec:

SELECT COUNT(DISTINCT name)

FROM MovieExec;

49

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.

50

Example: Effect of NULL’s

SELECT count(*)

FROM MovieExec;

SELECT count(name)

FROM MovieExec;

The number of tuplesin MovieExec.

The number of tuplesthat name is not NULLin MovieExec.

51

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.

52

Example: Grouping

From Movie, the sum of the lengths of all movies for each studio is expressed by:

SELECT studioName, SUM(length)

FROM Movie

GROUP BY studioName;

53

Example: Grouping

From Movie and MovieExec, find each producer’s total length of film produced.

SELECT name, SUM(length)

FROM MovieExec, Movie

WHERE producerC# = cert#

GROUP BY name;

54

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.

55

Illegal Query Example

You might think you could find the movie that is the longest in length:

SELECT title, MAX(length)

FROM Movie But this query is illegal in SQL.

56

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.

57

Example: HAVING

From Movie and MovieExec, find the total film length for only those producers who made at least one film prior to 1930.

SELECT name, SUM(length)

FROM MovieExec, Movie

WHERE producerC# = cert#

GROUP BY name

HAVING MIN(year) < 1930;

58

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.

59

练习

读者 ( 读者编号，姓名，电话 )图书 ( 书号 , 书名 , 作者 , 出版社 , 单价）借阅 ( 书号，读者编号，借阅日期 )

查询借阅过图书的读者姓名 查询图书总数 查询每天的图书借阅量（图书借阅记录数）