Chapter 4

Querying

Based on G. Post, DBMS: Designing & Building Business Applications

University of ManitobaAsper School of Business

3500 DBMSBob Travica

Updated 2015

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Why do we Need Queries

To extract meaningful data from database, which help us

answer business questions. Querying databases is where

business value from a DB system is drawn.

We need a standardized system so users and developers can

learn one method that works on any (most) systems. SQL vs. Query By Example (QBE)

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Four Questions to Create a Query

What data do you need to get? (columns in tables)

What table or tables are involved?

What are the constraints (filtering conditions for data,

search terms)?

If more tables needed, how to join them?

Download Sally’s Pet Store 2010 (queries work with it)

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SupplierIDNameContactNamePhoneAddressZipCodeCityID

Supplier

PONumberOrderDateReceiveDateSupplierIDEmployeeIDShippingCost

MerchandiseOrder

OrderIDOrderDateReceiveDateSupplierIDShippingCostEmployeeID

AnimalOrder

OrderIDAnimalIDCost

AnimalOrderItem

CityIDZipCodeCityStateAreaCodePopulation1990Population1980CountryLatitudeLongitude

City

EmployeeIDLastNameFirstNamePhoneAddressZipCodeCityIDTaxPayerIDDateHiredDateReleased

Employee

PONumberItemIDQuantityCost

OrderItem

CategoryRegistration

Category

CategoryBreed

Breed

AnimalIDNameCategoryBreedDateBornGenderRegisteredColorListPricePhoto

Animal

SaleIDSaleDateEmployeeIDCustomerIDSalesTax

Sale

SaleIDItemIDQuantitySalePrice

SaleItem

ItemIDDescriptionQuantityOnHandListPriceCategory

Merchandise

SaleIDAnimalIDSalePrice

SaleAnimal

CustomerIDPhoneFirstNameLastNameAddressZipCodeCityID

Customer

*

*

*

*

*

*

*

*

*

**

*

*

*

**

*

**

*

Your Querying Play Pen: Sally’s Pet Store Database

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Know your data!

Know your data!

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Sample Questions 1) List all animals with yellow in their

color.

2) List all dogs with yellow in their color

born after 6/1/01.

3) List all merchandise for cats with a list

price greater than $10.

4) List all dogs who are male and

registered or who were born before

6/1/01 and have white in their color.

5) What is the average sale price of all

animals?

6) What is the value of merchandise sold

per transaction record?

7) List the top 10 customers and total

amount they spent.

8) How many cats are in the animal list?

9) Show animals in each category with

the count above 10.

10) List the CustomerID of the

customers that purchased something

between 4/1/01 and 5/31/01.

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Query By Example vs. SQLQuery #1: List all animals with yellow in their color (only or along with other colors).

SELECT * FROM AnimalWHERE (Color LIKE “*yellow*”);

Which tables?

Which data(columns)

Which constraints

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SQL Query – Structure(Partial)

SELECT columns - What columns (information) to display?

FROM tables - What tables are involved?

(INNER JOIN - If more than one table used: What columns are the tables joined on -

PK & FK ?)

WHERE constraints - What are the constraints (filtering conditions) on records to be searched and displayed?

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More on Sample Query #1

Query question: List IDs, category, breed and color for animals that have yellow in their color.

Query statement:SELECTAnimalID, Category, Breed, ColorFROM AnimalWHERE Color LIKE “*yellow*”;

Variations of filtering condition – What do you get?:- WHERE Color like “yellow”, Color = “yellow”- WHERE Color LIKE [Please type color] - Application in parameter query (asking user to enter desired color)

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ORDER BY (Sorting)

SELECT columnsFROM tablesINNER JOIN columnsWHERE constraintsORDER BY columns ASC {or DESC}

SELECT Category, BreedFROM AnimalORDER BY Category, Breed; Category Breed

Bird ParakeetBird ParakeetBird ParrotBird ParrotBird ParrotCat AbyssinianCat AbyssinianCat American Short…

Output

Try to reverse Breed and Category in ORDER BY (and SELECT).

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Constraints

SELECTAnimalID, Category, Color, DateBornFROM AnimalWHERE ((Category="Dog") AND (Color Like "*Yellow*") AND (DateBorn > #6/1/2001#));

• Implemented as operators (Relational, Boolean, String)

String OperatorBoolean OperatorRelational Operator

Query #2: List all dogs with yellow in their color and born after 6/1/01.

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Query #3: List IDs and descriptions for the merchandise for cats with a list price greater than $10.

SQL statement:SELECT ItemID, Description, Category, ListPriceFROM Merchandise WHERE Category = "Cat" AND ListPrice > 10;

More on AND and > Operators

Output:

Note how the ListPrice constraint is presented as a number (no quotes).

ItemID Description Category ListPrice 5 Cat Bed-Small Cat 25,00 $ 6 Cat Bed-Medium Cat 35,00 $

14 Cat Food-Dry-25 pound Cat 18,00 $ 40 Litter Box-Covered Cat 15,00 $

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The AND vs. OR Operators

Query #4: List all dogs that are male and registered or that were born

before 6/1/2007 and have white in their color.

• Output will include some older non-registered dogs of either gender.• Note the Registered constraint (a null value = a blank cell, missing data).

- cannot be replaced by zero (0)- a reserved word in SQL syntax- can be understood as “we don’t know what the value is”• Best to write this query in two parts, test each separately, then

put them together.

SELECT AnimalID, Category, Gender, Registered, DateBorn, Color

FROM Animal

WHERE (Category="Dog" AND Gender="Male" AND Registered Is Not Null)

OR

(Category="Dog" AND DateBorn < #June 1, 2007# AND Color Like "*White*") ;

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Boolean Algebra

AND: True if both A and B exist.OR: True if either A or B or both exist.NOT: A (or B) does not exist.

2. Query with OR: (a > 4) OR (b < 0)

FF T

1. Query with AND:(a > 4) AND (b < 0) =(3 > 4) AND (-1 < 0)

F TT

3. Query with NOT: NOT (a > 4)

F

a = 3

b = -1

ExampleRecord

attributes a & b:

No match,Record not fethced

Match!

Match

A B A AND B A OR BT T T TT F F TF T F TF F F F

If ……….. Then…………………..…

-T NOT A F

F - NOT A T

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T

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BOO-lean Algebra

Parentheses indicate order of operations – start working from within parentheses, then work out with resulting values.

a = 7

b = 200

c = 2

F TF

T

( (a > 10) AND (b => 200) ) OR (c > 1)

T

1

2

3

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*

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DeMorgan’s LawCustomer: "I want to look at a cat, but I don’t want any cats that are registered or that have red in their color."

SELECT Category, Registered, Color FROM AnimalWHERE (Category="cat") AND

NOT ((Registered Is NOT NULL) OR (Color LIKE "*red*"));

More…

Category Registered Color

Cat Gray/Blue

Cat White

Cat White

Cat Yellow

Cat White

Used for simplifying cumbersome statements using NOT:

Output:

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Transformed into a simpler form by DeMorgan’s Law:

SELECT Category, Registered, Color FROM AnimalWHERE Category="cat" AND Registered Is Null AND

Color NOT LIKE "*red*";

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More…

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Negation of clauses NOT (A AND B) becomes NOT A OR NOT B, and the global NOT is

deleted NOT (A OR B) becomes NOT A AND NOT B, global NOT is deleted IS NOT becomes IS IS, = become NOT

T F

TF

NOT ((Registered IS NOT null) OR (Color LIKE “*red*”))

• Example: Constraints are Registered=ASCF, Color=Black

(Registered IS null) AND NOT (Color LIKE “*red*”)

FT

F

OR

NOT

AND

FNOT

DeMorgan’s Law applied:

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Computations – Row By Row

Query #6: What is the value of merchandise sold per transaction record?

SELECT SaleID, ItemID, SalePrice, Quantity, SalePrice*Quantity As SumFROM SaleItem;

SaleID ItemID SalePrice Quantity Sum 187 18 $0,90 50 45,00 $ 188 4 $58,50 1 58,50 $ 188 11 $27,00 6 162,00 $ 188 16 $0,72 25 18,00 $ 189 17 $0,45 30 13,50 $ 189 22 $67,50 2 135,00 $

Partial output:

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Computations On All Rows

Query #5: What is the average donation price of all animals?

SELECT Avg(Donation) AS [Average Donation Price] FROM Animal;

DBMS supports math & stats functions: Sum Avg Min Max Count StDev Var

Play with these functionsto learn more aboutanimals sold!

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Computations On Select Rows (with WHERE clause)

Query #8: How many cats are in the Animal list?

SELECT Count(AnimalID) AS [Cats Total]FROM AnimalWHERE Category = “Cat” ;

How would you make a general (parameter) query to find the count of animals in any category the user wants?

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Aggregate Functions – GROUP BY Clause

Arithmetic and statistical functions that work on an appropriate set of records.

Question: What is the average donation price per animal category?

The first idea is to build the query can on the previous, with addition of attribute Category: SELECT Category, Avg(Donation) AS [Average Donation Price] FROM Animal;

But DB system will report an error that Category is not part of an aggregate function.

Correct query is:

SELECT Category, Avg(Donation) AS [Average Donation Price]

FROM Animal

GROUP BY Category ;

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Computations On Select Rows (with HAVING clause)

Query #9: Show Categories of animals with the count above 10.

SELECT Category, Count(AnimalID) AS CountOfAnimalIDFROM AnimalGROUP BY CategoryHAVING Count(AnimalID) > 10ORDER BY Count(AnimalID) DESC;

optional

Processing order: (1) Group rows by attribute Category; (2) Find theaverage for each group; (3) Count rows in each group; (4) Comparethe counts with number 10; (5) select and display Category names andtheir counts where the count > 10.

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SQL Syntax(May Be Useful for later use)

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SELECTSELECT DISTINCT table.column {AS alias} , . . .FROM table/queryINNER JOIN table/query ON T1.ColA = T2.ColBWHERE (condition)GROUP BY columnHAVING (group condition)ORDER BY table.column{ UNION, INTERSECT, EXCEPT … }

GROUP BY CUBE (dimension1, dimension2, …)

TRANSFORM aggfunction {Crosstab values}SELECT . . . FROM . . . GROUP BY {Crosstab rows}PIVOT pivot column {Crosstab columns}

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CREATE TABLE

CREATE TABLE table( column1 datatype (size) [NOT NULL] [index1] , column2 datatype (size) [NOT NULL] [index2], … , CONSTRAINT pkname PRIMARY KEY (column, …), CONSTRAINT fkname FOREIGN KEY (column)

REFERENCES existing_table (key_column),)

See also:

ALTER TABLE

DROP TABLE

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ALTER TABLE

ALTER TABLE table ADD COLUMN column datatype (size) DROP COLUMN column

See also: CREATE TABLE DROP TABLE

SQL Syntax: COMMIT

COMMIT WORK

See also: ROLLBACK

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CREATE INDEX

CREATE [UNIQUE] INDEX indexON table (column1, column2, … )WITH {PRIMARY | DISALLOW NULL | IGNORE NULL}

See also: CREATE TABLE

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DROP

DROP INDEX index ON table

DROP TABLE

DROP VIEW

See also: DELETE

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DELETE

DELETEFROM tableWHERE condition

See also: DROP

INSERT

INSERT INTO table (column1, column2, …)VALUES (value1, value2, … )

INSERT INTO newtable (column1, column2, …)SELECT …

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UPDATE

UPDATE TABLE table SET column1 = value1, column2 = value2, … WHERE condition

See also: DELETE

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GRANT

GRANT privilege privilegesON object ALL, ALTER, DELETE, INDEX,TO user | PUBLIC INSERT, SELECT, UPDATE

See also: REVOKE

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REVOKE

REVOKE privilege privilegesON object ALL, ALTER, DELETE, INDEX,FROM user | PUBLIC INSERT, SELECT, UPDATE

See also: GRANT

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ROLLBACK

SAVEPOINT savepoint {optional}

ROLLBACK WORK TO savepoint

See also: COMMIT

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