2_introduction to sql.pdf

8/12/2019 2_Introduction to SQL.pdf

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Introduction to SQL

Select-From-Where Statements

Multirelation QueriesSubqueries


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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 detailsneeded in procedural languages like C++ orJava.

Database management system figures

out best way to execute query. Called query optimization.


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Select-From-Where Statements

SELECT desired attributesFROM one or more tables

WHERE condition about tuples ofthe tables


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Our Running Example

All our SQL queries will be based on thefollowing 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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Example

Using Beers(name, manf), what beers aremade by Anheuser-Busch?

SELECT name

FROM Beers

WHERE manf = Anheuser-Busch;


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Result of Query

nameBud

Bud LiteMichelob

. . .

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 FROMclause.

Apply the selection indicated by theWHERE clause.

Apply the extended projection indicated

by the SELECT clause.


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Operational Semantics

Check ifAnheuser-Busch

name manf

Bud Anheuser-Busch Include t.namein the result, if so

Tuple-variable tloops over all

tuples


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Operational Semantics --- General

Think of a tuple variable visiting eachtuple of the relation mentioned in FROM.

Check if the current tuple satisfies theWHERE clause.

If so, compute the attributes or

expressions of the SELECT clause usingthe components of this tuple.


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* In SELECT clauses

When there is one relation in the FROMclause, * 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-BuschMichelob Anheuser-Busch

. . . . . .

Now, the result has each of the attributesof Beers.


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Renaming Attributes

If you want the result to have differentattribute names, use AS torename an attribute.

Example: Using Beers(name, manf):

SELECT name AS beer, manf

FROM BeersWHERE manf = Anheuser-Busch


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Result of Query:

beer manf Bud Anheuser-Busch

Bud Lite Anheuser-BuschMichelob Anheuser-Busch

. . . . . .


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Expressions in SELECT Clauses

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

Example: Using Sells(bar, beer, price):

SELECT bar, beer,

price*114 AS priceInYen

FROM Sells;


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Result of Query

bar beer priceInYenJoes Bud 285

Sues Miller 342


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Example: Constants as Expressions

Using Likes(drinker, beer):

SELECT drinker,likes Bud AS whoLikesBud

FROM Likes

WHERE beer = Bud;


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Result of Query

drinker whoLikesBudSally likes Bud

Fred likes Bud


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Example: Information Integration

We often build data warehouses fromthe data at many sources.

Suppose each bar has its own relation

Menu(beer, price) .

To contribute to Sells(bar, beer, price)

we need to query each bar and insertthe name of the bar.


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Information Integration --- (2)

For instance, at Joes Bar we can issuethe query:

SELECT Joes Bar, beer, price

FROM Menu;


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Complex Conditions in WHEREClause

Boolean operators AND, OR, NOT.Comparisons =, , , =.

And many other operators that produceboolean-valued results.


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Example: Complex Condition

Using Sells(bar, beer, price), find the priceJoes Bar charges for Bud:

SELECT price

FROM Sells

WHERE bar = Joes Bar AND

beer = Bud;


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Patterns

A condition can compare a string to apattern by:

LIKE or

NOT LIKE

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


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Example: LIKE

Using Drinkers(name, addr, phone) findthe drinkers with exchange 555:

SELECT name

FROM Drinkers

WHERE phone LIKE %555-_ _ _ _;


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NULL Values

Tuples in SQL relations can have NULLas a value for one or more components.

Meaning depends on context. Two

common cases:Missing value: e.g., we know Joes Bar has

some address, but we dont know what it is.

Inapplicable: e.g., the value of attributespouse for an unmarried person.


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Comparing NULLs to Values

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

Comparing any value (including NULL

itself) with NULL yields UNKNOWN.

A tuple is in a query answer iff the

WHERE clause is TRUE (not FALSE orUNKNOWN).


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Three-Valued Logic

To understand how AND, OR, and NOTwork in 3-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, ) = .


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Surprising Example

From the following Sells relation:bar beer price

Joes Bar Bud NULL

SELECT bar

FROM Sells

WHERE price < 2.00 OR price >= 2.00;UNKNOWN UNKNOWN

UNKNOWN


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

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

But not others, e.g., the law of the

excluded middle :pOR NOTp= TRUE.Whenp= UNKNOWN, the left side is

MAX( , (1 )) = != 1.


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Multirelation Queries

Interesting queries often combine datafrom more than one relation.

We can address several relations in one

query by listing them all in the FROMclause.

Distinguish attributes of the same nameby . .


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Example: Joining Two RelationsUsing relations Likes(drinker, beer) and

Frequents(drinker, bar), find the beers likedby at least one person who frequents JoesBar.

SELECT beer

FROM Likes, Frequents

WHERE bar = Joes Bar AND

Frequents.drinker =

Likes.drinker;


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Formal Semantics

Almost the same as for single-relationqueries:

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 andexpressions in the SELECT clause.


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Operational Semantics

Imagine one tuple-variable for eachrelation in the FROM clause.

These tuple-variables visit each

combination of tuples, one from eachrelation.

If the tuple-variables are pointing to

tuples that satisfy the WHERE clause,send these tuples to the SELECT clause.


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Example

drinker bar drinker beer

tv1 tv2Sally Bud

Sally Joes

Likes

Frequentsto outputcheck these

are equal

checkfor Joe


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Explicit Tuple-Variables

Sometimes, a query needs to use twocopies of the same relation.

Distinguish copies by following the

relation name by the name of a tuple-variable, in the FROM clause.

Its always an option to renamerelations this way, even when notessential.


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Example: Self-Join

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).SELECT b1.name, b2.name

FROM Beers b1, Beers b2

WHERE b1.manf = b2.manf AND

b1.name < b2.name;


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SubqueriesA parenthesized SELECT-FROM-WHERE

statement (subquery) can be used as avalue in a number of places, includingFROM and WHERE clauses.

Example: in place of a relation in theFROM clause, we can use a subquery

and then query its result.Must use a tuple-variable to name tuples of

the result.


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Example: Subquery in FROM

Find the beers liked by at least one personwho frequents Joes Bar.

SELECT beer

FROM Likes, (SELECT drinker

FROM Frequents

WHERE bar = Joes Bar)JDWHERE Likes.drinker = JD.drinker;

Drinkers whofrequent Joes Bar


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Subqueries That Return One Tuple

If a subquery is guaranteed to produceone tuple, then the subquery can beused 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: Single-Tuple Subquery

Using Sells(bar, beer, price), find thebars that serve Miller for the same priceJoe 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 barFROM Sells

WHERE beer = Miller AND

price = (SELECT price

FROM Sells

WHERE bar = Joes BarAND beer = Bud);

The price atwhich Joesells Bud


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The IN Operator

IN () is true if andonly if the tuple is a member of therelation produced by the subquery.

Opposite: NOT IN ().

IN-expressions can appear in WHEREclauses.


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Example: IN

Using Beers(name, manf) and Likes(drinker,

beer), find the name and manufacturer ofeach beer that Fred likes.

SELECT *

FROM Beers

WHERE name IN (SELECT beer

FROM LikesWHERE drinker = Fred);

The set ofbeers Fredlikes


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Remember These From Lecture #1?

SELECT aFROM R, S

WHERE R.b = S.b;

SELECT a

FROM R

WHERE b IN (SELECT b FROM S);


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IN is a Predicate About Rs Tuples

SELECT aFROM R

WHERE b IN (SELECT b FROM S);

One loop, overthe tuples of R

a b1 2

3 4R

b c2 5

2 6S

(1,2) satisfiesthe condition;

1 is output once.

Two 2s


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This Query Pairs Tuples from R, S

SELECT aFROM R, S

WHERE R.b = S.b;

Double loop, overthe tuples of R and S

a b1 2

3 4R

b c2 5

2 6S

(1,2) with (2,5)and (1,2) with

(2,6) both satisfythe condition;1 is output twice.


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The Exists Operator

EXISTS() is true if and onlyif the subquery result is not empty.

Example: From Beers(name, manf) ,

find those beers that are the uniquebeer by their manufacturer.


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Example: EXISTS

SELECT nameFROM Beers b1

WHERE NOT EXISTS (

SELECT *

FROM Beers

WHERE manf = b1.manf ANDname b1.name);

Set ofbeerswith thesamemanf asb1, butnot thesame

beer

Notice scope rule: manf refers

to closest nested FROM witha relation having that attribute.

Notice theSQL notequalsoperator


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The Operator ANY

x= ANY() is a booleancondition that is true iffxequals at leastone tuple in the subquery result.

= could be any comparison operator.

Example: x>= ANY() means xis not the uniquely smallest tuple producedby the subquery.

Note tuples must have one component only.


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Example: ALL

From Sells(bar, beer, price), find thebeer(s) sold for the highest price.

SELECT beer

FROM Sells

WHERE price >= ALL(

SELECT priceFROM Sells);

price from the outerSells must not beless than any price.


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Union, Intersection, and Difference

Union, intersection, and difference ofrelations are expressed by the followingforms, each involving subqueries:

() UNION () () INTERSECT ()

() EXCEPT ()


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Example: Intersection

Using Likes(drinker, beer), Sells(bar, beer,price), and Frequents(drinker, bar), findthe drinkers and beers such that:

1. The drinker likes the beer, and2. The drinker frequents at least one bar that

sells the beer.

Notice trick:


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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 the

beer.

subquery is

really a storedtable.


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Bag Semantics

Although the SELECT-FROM-WHEREstatement uses bag semantics, thedefault 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, it is easier toavoid eliminating 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 bySELECT DISTINCT . . .

Force the result to be a bag (i.e., dont

eliminate duplicates) by ALL, as in. . . UNION ALL . . .


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Example: DISTINCT

From Sells(bar, beer, price), find all thedifferent prices charged for beers:

SELECT DISTINCT price

FROM Sells;

Notice that without DISTINCT, each

price would be listed as many times asthere were bar/beer pairs at that price.


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Example: ALL

Using relations Frequents(drinker, bar) andLikes(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 timesas the difference of those counts.


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Join Expressions

SQL provides several versions of (bag)joins.

These expressions can be stand-alone

queries or used in place of relations in aFROM 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 JoinR JOIN S ON

Example: using Drinkers(name, addr) andFrequents(drinker, bar):

Drinkers JOIN Frequents ON

name = drinker;

gives us all (d, a, d, b) quadruples such

that drinker d lives at address a andfrequents bar b.

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