1

CSCE 608-600 Database Systems

Anxiao (Andrew) Jiang

The Database Language SQL

2

Introduction to SQL

Select-From-Where Statements

Subqueries

Grouping and Aggregation

3

Select-From-Where Statements

SELECT desired attributes

FROM one or more tables

WHERE condition about tuples of

the tables

4

Our Running Example

All our SQL queries will be based on the following database schema. Underline indicates key attributes.

Candies(name, manf)Stores(name, addr, license)Consumers(name, addr, phone)Likes(consumer, candy)Sells(store, candy, price)Frequents(consumer, store)

5

Example

Using Candies(name, manf), what candies are made by Hershey?

SELECT name

FROM Candies

WHERE manf = ’Hershey’;

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

6

Result of Query

name

Twizzler

Kitkat

AlmondJoy

. . .

The answer is a relation with a single attribute,name, and tuples with the name of each candyby Hershey, such as Twizzler.

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 (tv) 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

Operational Semantics

Check ifHershey

name manf

Twizzler Hershey

tv

Include tv.name in the result

10

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>”

11

Example

Using relations Likes(consumer, candy) and Frequents(consumer, store), find the candies liked by at least one person who frequents 7-11.

SELECT candyFROM Likes, FrequentsWHERE store = ’7-11’ AND

Frequents.consumer = Likes.consumer;

12

Formal Semantics

Almost the same as for single-relation queries:

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

Apply the selection condition from the WHERE clause.

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

13

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.

14

Example

From Candies(name, manf), find all pairs of candies by the same manufacturer. Do not produce pairs like (Twizzler, Twizzler). Produce pairs in alphabetic order, e.g. (Kitkat,

Twizzler), not (Twizzler, Kitkat).

SELECT c1.name, c2.name

FROM Candies c1, Candies c2

WHERE c1.manf = c2.manf AND

c1.name < c2.name;

tuplevariables

15

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.Can use a tuple-variable to name tuples of

the result.

16

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.

17

Example

From Sells(store, candy, price), find the stores that sell Kitkats for the same price 7-11 charges for Twizzlers.

Two queries would surely work: Find the price 7-11 charges for Twizzlers. Find the stores that sell Kitkats at that

price.

18

Query + Subquery Solution

SELECT storeFROM SellsWHERE candy = ’Kitkat’ AND

price = (SELECT price FROM Sells WHERE store= ’7-11’

AND candy = ’Twizzler’);

The price atwhich 7-11sells Twizzlers

19

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.

20

Example

From Candies(name, manf) and Likes(consumer, candy), find the name and manufacturer of each candy that Fred likes.

SELECT *

FROM Candies

WHERE name IN (SELECT candy

FROM Likes

WHERE consumer = ’Fred’);

The set ofcandies Fredlikes

21

The Exists Operator

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

Example: From Candies(name, manf) , find those candies that are the unique candy by their manufacturer.

22

Example Query with EXISTS

SELECT nameFROM Candies c1WHERE NOT EXISTS(

SELECT *FROM CandiesWHERE manf = c1.manf AND

name <> c1.name);

Set ofcandieswith thesamemanf asc1, butnot thesamecandy

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

23

The Operator ANY

x = ANY( <relation> ) is a boolean condition that is 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.

24

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.

25

Example

From Sells(store, candy, price), find the candies sold for the highest price.

SELECT candy

FROM Sells

WHERE price >= ALL(

SELECT price

FROM Sells);

price from the outerSells must not beless than any price.

26

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 )

27

Example

From relations Likes(consumer, candy), Sells(store, candy, price), and Frequents(consumer, store), find the consumers and candies such that:

The consumer likes the candy, and The consumer frequents at least one

store that sells the candy.

28

Solution

(SELECT * FROM Likes)

INTERSECT

(SELECT consumer, candy

FROM Sells, Frequents

WHERE Frequents.store = Sells.store

);

The consumer frequentsa store that sells thecandy.

29

Bag Semantics

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.

30

Motivation: Efficiency

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

duplicates anyway.

31

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 . . .

32

Example: DISTINCT

From Sells(store, candy, price), find all the different prices charged for candies:

SELECT DISTINCT priceFROM Sells;

Notice that without DISTINCT, each price would be listed as many times as there were store/candy pairs at that price.

33

Example: ALL

Using relations Frequents(consumer, store) and Likes(consumer, candy):

(SELECT consumer FROM Frequents)

EXCEPT ALL

(SELECT consumer FROM Likes); Lists consumers who frequent more stores than

they like candies, and does so as many times as the difference of those counts.

34

Join Expressions

SQL provides several versions of (bag) joins.

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

35

Products and Natural Joins

Natural join:

R NATURAL JOIN S; Product:

R CROSS JOIN S; Example:

Likes NATURAL JOIN Sells; Relations can be parenthesized subqueries, as

well.

36

Theta Join

R JOIN S ON <condition> Example: using Consumers(name, addr)

and Frequents(consumer, store):Consumers JOIN Frequents ON

name = consumer;gives us all (c, a, c, s) quadruples such that consumer c lives at address a and frequents store s.

37

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.

38

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.

39

Example: Aggregation

From Sells(store, candy, price), find the average price of Twizzlers:

SELECT AVG(price)

FROM Sells

WHERE candy = ’Twizzler’;

40

Eliminating Duplicates in an Aggregation

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

prices charged for Twizzlers:

SELECT COUNT(DISTINCT price)

FROM Sells

WHERE candy = ’Twizzler’;

41

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.

42

Example: Effect of NULL’s

SELECT count(*)FROM SellsWHERE candy = ’Twizzler’;

SELECT count(price)FROM SellsWHERE candy = ’Twizzler’;

The number of storesthat sell Twizzlers.

The number of storesthat sell Twizzlers at aknown price.