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CS 464 /CS 564

INTRODUCTION TO

DATABASE MANAGEMENTLecturer: Lydia Tapia

Lecture Eight: Introduction to SQL

http://www.cs.unm.edu/~tapia/cs464_cs564/

Source: Slides by Jeffrey Ullman

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INTRODUCTION TO SQL

Select-From-Where Statements

Subqueries

Grouping and Aggregation

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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 outbest way to execute query.

Called query optimization.

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Select-From-Where Statements

SELECT desired attributes

FROM one or more tables

WHERE condition about tuples of

the tables

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Our Running ExampleAll our SQL queries will be based on the followingdatabase 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)

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ExampleUsing Candies(name, manf), what candies aremade by Hershey?

SELECT nameFROM Candies

WHERE manf = Hershey;

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

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Result of Queryname

Twizzler

KitkatAlmondJoy

. . .

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

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Meaning of Single-Relation Query

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

Apply the extended projection indicated by theSELECT clause.

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Operational Semantics

To implement this algorithm think of a tuplevariable (tv) ranging over each tuple of the

relation mentioned in FROM.Check if the current tuple satisfies theWHERE clause.

If so, compute the attributes or expressions ofthe SELECT clause using the components ofthis tuple.

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Check if

Hershey

name manf

Twizzler Hersheytv

Include tv.namein the result

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* In SELECT clauses

When there is one relation in the FROM clause, *in the SELECT clause stands forall attributes of

this relation.

Example using Candies(name, manf):SELECT *

FROM Candies

WHERE manf = Hershey;

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Result of Query:

name manf

Twizzler Hershey

Kitkat Hershey

AlmondJoy Hershey

. . . . . .

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Now, the result has each of the attributesof Candies.

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Renaming Attributes

If you want the result to have different attributenames, use AS to rename an

attribute.Example based on Candies(name, manf):

SELECT name AS candy, manf

FROM Candies

WHERE manf = Hershey

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Result of Query:

candy manf

Twizzler Hershey

Kitkat HersheyAlmondJoy Hershey

. . . . . .

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Expressions in SELECT Clauses

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

Example: from Sells(store, candy, price):SELECT store, candy,

price * 79 AS priceInYen

FROM Sells;

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Result of Query

store candy priceInYen

7-11 Twizzler 285

Smiths Snickers 342

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Another Example: Constant Expressions

From Likes(consumer, candy) :

SELECT consumer,likes Kitkats

AS whoLikesKitkats

FROM Likes

WHERE candy =Kitkat

;

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Result of Query

consumer whoLikesKitkats

Sally likes Kitkats

Fred likes Kitkats

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Complex Conditions in WHERE Clause

From Sells(store, candy, price), find the price that7-11 charges for Twizzlers:

SELECT price

FROM Sells

WHERE store = 7-11 AND

candy = Twizzler;

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Patterns

WHERE clauses can have conditions inwhich a string is compared with a pattern,

to see if it matches.

General form: LIKE or NOT LIKE

Pattern is a quoted string with % = anystring; _ = any character.

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Example

From Consumers(name, addr, phone) find theconsumers with exchange 555:

SELECT name

FROM Consumers

WHERE phone LIKE%555-_ _ _ _

;

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NULL Values

Tuples in SQL relations can have NULL as avalue for one or more components.

Meaning depends on context. Two commoncases:

Missing value: e.g., we know 7-11 has someaddress, but we dont know what it is.

Inapplicable : e.g., the value of attribute spouse foran unmarried person.

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Comparing NULLs to Values

The logic of conditions in SQL is really 3-valuedlogic: TRUE, FALSE, UNKNOWN.

When any value is compared with NULL, thetruth value is UNKNOWN.

But a query only produces a tuple in the answer ifits 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 in3-valued logic, think of TRUE = 1, FALSE = 0,

and UNKNOWN = .

AND = MIN; OR = MAX, NOT(x) = 1-x.Example:TRUE AND (FALSE OR NOT(UNKNOWN)) =

MIN(1, MAX(0, (1 - ))) =

MIN(1, MAX(0, ) = MIN(1, ) = =

UNKNOWN.

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Surprising Example

From the following Sells relation:store candy price

7-11 Twizzler NULL

SELECT store

FROM Sells

WHERE price < 2.00 OR price >= 2.00;

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UNKNOWN UNKNOWN

UNKNOWN

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

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

But not others, e.g., the law of the excludedmiddle:p OR NOTp = TRUE.

Whenp = UNKNOWN, the left side is MAX( , (1 )) = != 1.

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Multirelation Queries

Interesting queries often combine data from morethan one relation.

We can address several relations in one query bylisting them all in the FROM clause.

Distinguish attributes of the same name by.

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Example

Using relations Likes(consumer, candy) andFrequents(consumer, store), find the candies likedby at least one person who frequents 7-11.

SELECT candyFROM Likes, Frequents

WHERE store = 7-11 AND

Frequents.consumer =

Likes.consumer;

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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 WHEREclause.

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

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Imagine one tuple-variable for each relation in theFROM clause.

These tuple-variables visit each combination of tuples,one from each relation.

If the tuple-variables are pointing to tuples thatsatisfy the WHERE clause, send these tuples to

the SELECT clause.

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Example

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consumer store consumer candy

tv1 tv2Sally Twizzler

Sally 7-11

LikesFrequents

to outputcheck theseare equal

check

for 7-11

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Explicit Tuple-Variables

Sometimes, a query needs to use two copiesof the same relation.

Distinguish copies by following the relationname by the name of a tuple-variable, in the

FROM clause.

Its always an option to rename relations thisway, even when not essential.

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Example

From Candies(name, manf), find all pairs ofcandies 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 c2WHERE c1.manf = c2.manf AND

c1.name < c2.name;

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tuplevariables

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Subqueries

A parenthesized SELECT-FROM-WHEREstatement (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 FROMclause, we can place another query, and then

query its result.

Can use a tuple-variable to name tuples of theresult.

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Subqueries That Return One Tuple

If a subquery is guaranteed to produce onetuple, 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.

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Example

From Sells(store, candy, price), find the storesthat 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.

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Query + Subquery Solution

SELECT store

FROM Sells

WHERE candy = Kitkat AND

price = (SELECT price

FROM Sells

WHERE store= 7-11

AND candy = Twizzler);

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The price atwhich 7-11

sells Twizzlers

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The IN Operator

IN is true if and only if thetuple is a member of the relation. NOT IN means the opposite.

IN-expressions can appear in WHERE clauses.The is often a subquery.

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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 LikesWHERE consumer =Fred);

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The set of

candies Fredlikes

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The Exists Operator

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

Example: From Candies(name, manf) , findthose candies that are the unique candy bytheir manufacturer.

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Example Query with EXISTS

SELECT name

FROM Candies c1

WHERE NOT EXISTS(

SELECT *

FROM Candies

WHERE manf = c1.manf AND

name c1.name);

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Set ofcandieswith thesamemanf as

c1, butnot thesamecandy

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

Notice theSQL notequalsoperator

From Candies(name, manf) ,find those candies that are

the unique candy by their

manufacturer.

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The Operator ANY

x= ANY( ) is a boolean condition trueifxequals at least one tuple in the relation.

Similarly, = can be replaced by any of thecomparison operators.

Example:x> ANY( ) meansxis notthe smallest tuple in the relation.

Note tuples must have one component only.

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The Operator ALL

Similarly,x ALL( ) is true if andonly if for every tuple t in the relation,xis not

equal to t. That is,xis not a member of the relation.

The can be replaced by any comparisonoperator.

Example:x>= ALL( ) means thereis no tuple larger thanx in the relation.

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Example

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

SELECT candy

FROM Sells

WHERE price >= ALL(

SELECT price

FROM Sells);

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price from the outerSells must not beless than any price.

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Union, Intersection, and Difference

Union, intersection, and difference of relations areexpressed by the following forms, each involving

subqueries:

( subquery ) UNION ( subquery ) ( subquery ) INTERSECT ( subquery ) ( subquery ) EXCEPT ( subquery )

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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.

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Solution

(SELECT * FROM Likes)

INTERSECT

(SELECT consumer, candyFROM Sells, Frequents

WHERE Frequents.store = Sells.store

);

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The consumer frequentsa store that sells thecandy.

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Bag Semantics

Although the SELECT-FROM-WHEREstatement uses bag semantics, the default for

union, intersection, and difference is set

semantics.

That is, duplicates are eliminated as the operation isapplied.

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Motivation: Efficiency

When doing projection, it is easier to avoideliminating duplicates.

Just work tuple-at-a-time.For intersection or difference, it is mostefficient to sort the relations first.

At that point you may as well eliminate theduplicates anyway.

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Controlling Duplicate Elimination

Force the result to be a set by SELECTDISTINCT . . .

Force the result to be a bag (i.e., donteliminate duplicates) by ALL, as in . . .

UNION ALL . . .

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Example: DISTINCT

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

SELECT DISTINCT price

FROM Sells;Notice that without DISTINCT, each price wouldbe listed as many times as there were store/candy pairs at that price.

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Example: ALL

Using relations Frequents(consumer, store) andLikes(consumer, candy):

(SELECT consumer FROM Frequents)EXCEPT ALL

(SELECT consumer FROM Likes);

Lists consumers who frequent more stores thanthey like candies, and does so as many times asthe difference of those counts.

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Join Expressions

SQL provides several versions of (bag) joins.These expressions can be stand-alone queries orused in place of relations in a FROM clause.

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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, aswell.

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Theta Join

R JOIN S ON Example: using Consumers(name, addr) andFrequents(consumer, store):

Consumers JOIN Frequents ON

name = consumer;

gives us all (c, a, c, s) quadruples such that

consumerclives at address a and frequents stores.

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Outerjoins

R OUTER JOIN S is the core of an outerjoinexpression. It is modified by:

1. Optional NATURAL in front of OUTER.2. Optional ON 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 beapplied to a column in a SELECT clause to

produce that aggregation on the column.

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

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Example: Aggregation

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

SELECT AVG(price)

FROM Sells

WHERE candy = Twizzler;

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Eliminating Duplicates in an Aggregation

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

SELECT COUNT(DISTINCT price)

FROM Sells

WHERE candy =Twizzler

;

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NULLs Ignored in Aggregation

NULL never contributes to a sum, average, orcount, and can never be the minimum or

maximum of a column.

But if there are no non-NULL values in acolumn, then the result of the aggregation is

NULL.

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Example: Effect of NULLs

SELECT count(*)

FROM Sells


SELECT count(price)

FROM Sells


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The number of storesthat sell Twizzlers.

The number of storesthat sell Twizzlers at aknown price.

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Grouping

We may follow a SELECT-FROM-WHEREexpression by GROUP BY and a list of attributes.

The relation that results from the SELECT-FROM-WHERE is grouped according to the values of allthose attributes, and any aggregation is applied

only within each group.

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Example: Grouping

From Sells(store, candy, price), find the averageprice for each candy:

SELECT candy, AVG(price)

FROM Sells

GROUP BY candy;

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Example: Grouping

From Sells(store, candy, price) and Frequents(consumer, store), find for each consumer the

average price of Twizzlers at the stores they

frequent:SELECT consumer, AVG(price)

FROM Frequents, Sells

WHERE candy = Twizzler AND

Frequents.store = Sells.store

GROUP BY consumer;

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Computeconsumer-store-pricefor Twiz.tuples first,then group

by consumer.

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Restriction on SELECT Lists With Aggregation

If any aggregation is used, then eachelement of the SELECT list must be either:

1. Aggregated, or2. An attribute on the GROUP BY list.

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Query Example Correct or Not?

You might think you could find the store thatsells Twizzlers the cheapest by:

SELECT store, MIN(price)FROM Sells


But this query is illegal in SQL.

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HAVING Clauses

HAVING may follow a GROUP BYclause.

If so, the condition applies to each group, andgroups not satisfying the condition are eliminated.

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Example: HAVING

From Sells(store, candy, price) and Candies(name, manf), find the average prices of those

candies that are either sold in at least three stores

or are manufactured by Nestle.

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Solution

SELECT candy, AVG(price)

FROM Sells

GROUP BY candyHAVING COUNT(store) >= 3 OR

candy IN (SELECT name

FROM Candies

WHERE manf = Nestle);

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Candiesmanufactured

by Nestle.

Candy groups with at least3 non-NULL stores and alsocandy groups where themanufacturer is Nestle.

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Requirements on HAVING Conditions

These conditions may refer to any relation ortuple-variable in the FROM clause.

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

A grouping attribute, or Aggregated.

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