More on Structured Query Language Indra Budi [email protected]

More onStructured Query Language

Indra [email protected]

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Join ExpressionJoin Expression

Permitting user to specify a table resulting from join operation in FROM clause of a queryTypes of join:

Cross join (cross product)Natural joinTheta joinOuter join



Cross joinJoin two tables without any conditionsSimilar to concept of Cartesian product in set conceptSQL syntax:

<relation> CROSS JOIN <relation>

Natural joinRequiring the two join attributes have the same attribute namesChange the attribute name if the attributes have different nameThe resulting relation contains all attributes from the two tables, but only one join attribute is keptSQL syntax:

<relation> NATURAL JOIN <relation>



Theta joinJoining two related tables using join condition in FROM clauseThe resulting relation contains all attributes from the two tables, including the attribute(s) for join conditionSQL Syntax:

<relation> JOIN <relation> ON <condition>


Join ExpressionJoin ExpressionOuter join

Similar to theta join, the difference:• Theta join only returns the tuples with a matching tuple (relation)• Outer join returns all tuples with or without matching tuple

(relation)

Types: Left, right and full outer joinSQL Syntax:

<relation1> OUTER JOIN <relation2> 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 relation1 only.

RIGHT = pad dangling tuples of relation2 only.

FULL = pad both; this choice is the default.


Example-Cross JoinExample-Cross Join

Beers

name manf

Beer 1 XYZ

Beer 2 ABC

Beer 3 ABC

Likes

drinker

beer

Andika Beer 1

Panca Beer 1

Mahesa

Beer 3

SELECT * FROM Beers CROSS JOIN Likes

name manf drinker Beer

Beer 1 XYZ Andika Beer 1

Beer 1 XYZ Panca Beer 1

Beer 1 XYZ Mahesa Beer 3

Beer 2 ABC Andika Beer 1

Beer 2 ABC Panca Beer 1

Beer 2 ABC Mahesa Beer 3

Beer 3 ABC Andika Beer 1

Beer 3 ABC Panca Beer 1

Beer 3 ABC Mahesa Beer 3


Example-Natural JoinExample-Natural JoinLikes

drinker

beer

Andika Beer 1

Mahesa

Beer 1

Panca Beer 3

Siti Beer 2

Frequents

drinker bar

Tyas ABC

Bambang XYZ

Panca ABC

Andika Zanz

SELECT * FROM Beers NATURAL JOIN Likes

drinker beer bar

Andika Beer 1 Zanz

Panca Beer 3 ABC


Example-Theta JoinExample-Theta Join

Beers

name manf

Beer 1 XYZ

Beer 2 ABC

Beer 3 ABC

Likes

drinker

beer

Aryo Beer 1

Lamo Beer 1

Amir Beer 3

SELECT * FROM Beers B JOIN Likes L ON B.name = L.beer

name manf drinker beer

Beer 1 XYZ Aryo Beer 1

Beer 1 XYZ Lamo Beer 1

Beer 3 ABC Amir Beer 3


Example-Outer JoinExample-Outer Join

Beers

name manf

Beer 1 XYZ

Beer 2 ABC

Beer 3 ABC

Likes

SELECT * FROM Beers B LEFT OUTER JOIN Likes L ON B.name = L.beername manf drinker beer


Beer 1 XYZ Amir Beer 1

Beer 2 ABC

Beer 3 ABC Fitria Beer 3

drinker

beer

Aryo Beer 1

Amir Beer 1

Siti Beer 3

Fitria Beer 5

SELECT * FROM Beers B RIGHT OUTER JOIN Likes L ON B.name = L.beername manf drinker beer



Beer 3 ABC Siti Beer 3

Fitria Beer 5


Example-Outer JoinExample-Outer Join

Beers

name manf

Beer 1 XYZ

Beer 2 ABC

Beer 3 ABC

Likes

SELECT * FROM Beers B FULL OUTER JOIN Likes L ON B.name = L.beername manf drinker beer



Beer 2 ABC

Beer 3 ABC Siti Beer 3

Fitria Beer 5

drinker

beer

Aryo Beer 1

Amir Beer 1

Siti Beer 3

Fitria Beer 5


AggregationsAggregations

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.


Example: AggregationExample: Aggregation

From Sells(bar, beer, price), find the average price of Bud:

SELECT AVG(price)FROM SellsWHERE beer = ‘Bud’;

From Sells(bar, beer, price), find the cheapest and the most expensive price of Bud:

SELECT MIN(price), MAX(price)FROM SellsWHERE beer = ‘Bud’;


Eliminating Duplicates in an AggregationEliminating Duplicates in an Aggregation

DISTINCT inside an aggregation causes duplicates to be eliminated before the aggregation.Example: find the number of different prices charged for Bud:

SELECT COUNT(DISTINCT price)

FROM Sells

WHERE beer = ‘Bud’;


ExampleExample

Sells

bar beer price

ABC Beer 1 5

ABC Beer 2 8

XYZ Beer 3 5

FGH Beer 2 7

WXY Beer 2 7

SELECT AVG(price) as average_priceFROM SellsWHERE beer = ‘Beer 2’;

average_price

7.33

SELECT AVG(DISTINCT price) as average_priceFROM SellsWHERE beer = ‘Beer 2’;

average_price

7.5


NULL’s Ignored in AggregationNULL’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 (all values are NULL), then the result of the aggregation is NULL.


Example: Effect of NULL’sExample: Effect of NULL’s

SELECT count(*)FROM SellsWHERE beer = ‘Bud’;

The number of barsthat sell Bud.

SELECT count(price)FROM SellsWHERE beer = ‘Bud’;

The number of barsthat sell Bud at aknown price (price not NULL)

bar beer price

ABC Beer 1 5

ABC Bud 8

XYZ Beer 3 5

FGH Bud 7

WXY Bud

The result : 3

The result : 2


GroupingGrouping

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.


Example: GroupingExample: Grouping

From Sells(bar, beer, price), find the average price for each beer:

SELECT beer, AVG(price)

FROM Sells

GROUP BY beer;

From Sells(bar, beer, price), find the cheapest price for each beer:

SELECT beer, MIN(price)

FROM Sells

GROUP BY beer;


Example: GroupingExample: Grouping

From Sells(bar, beer, price) and Frequents(drinker, bar), find for each drinker the average price of Bud at the bars they frequent:

SELECT drinker, AVG(price)FROM Frequents, SellsWHERE beer = ‘Bud’ AND

Frequents.bar = Sells.barGROUP BY drinker;

Computedrinker-bar-price of Budtuples first,then groupby drinker.


Restriction on SELECT Lists With AggregationRestriction on SELECT Lists With Aggregation

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

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


Illegal Query ExampleIllegal Query Example

You might think you could find the bar that sells Bud the cheapest by:

SELECT bar, MIN(price)FROM SellsWHERE beer = ‘Bud’;

But this query is illegal in SQL.Why? Note bar is neither aggregated nor on the GROUP BY list.


HAVING ClausesHAVING Clauses

HAVING vs WHEREHAVING filtering groupWHERE filtering tuples

HAVING <condition> may follow a GROUP BY clause.If so, the condition applies to each group, and groups not satisfying the condition are eliminated.


Requirements on HAVING ConditionsRequirements 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, or2. Aggregated.


Example: HAVINGExample: HAVING

From Sells(bar, beer, price), find the beer name and cheapest price for that beer ranging from 4 to 10.

SELECT beer, MIN(price)

FROM SellsGROUP BY beerHAVING MIN(price) >= 4 AND MIN(price) <=10


Example: HAVINGExample: HAVING

From Sells(bar, beer, price) and Beers(name, manf), find the average price of those beers that are either served in at least three bars or are manufactured by Pete’s.


SolutionSolution

SELECT beer, AVG(price)FROM SellsGROUP BY beerHAVING COUNT(bar) >= 3 OR

beer IN (SELECT name FROM Beers WHERE manf = ‘Pete’’s’);

Beers manu-factured byPete’s.

Beer groups with at least3 non-NULL bars and alsobeer groups where themanufacturer is Pete’s.


Database ModificationsDatabase Modifications

A modification command does not return a result as a query does, but it changes the database in some way.There are three kinds of modifications:

1. Insert a tuple or tuples.2. Delete a tuple or tuples.3. Update the value(s) of an existing tuple or tuples.


InsertionInsertion

To insert a single tuple:INSERT INTO <relation>VALUES ( <list of values> );

Example: add to Likes(drinker, beer) the fact that Sally likes Bud.

INSERT INTO Likes

VALUES(‘Sally’, ‘Bud’);


Specifying Attributes in INSERTSpecifying Attributes in INSERT

We may add to the relation name a list of attributes.There are two reasons to do so:

1. We forget the standard order of attributes for the relation.

2. We don’t have values for all attributes, and we want the system to fill in missing components with NULL or a default value.


Example: Specifying AttributesExample: Specifying Attributes

Another way to add the fact that Sally likes Bud to Likes(drinker, beer):

INSERT INTO Likes(beer, drinker)

VALUES(‘Bud’, ‘Sally’);


Inserting Many TuplesInserting Many Tuples

We may insert the entire result of a query into a relation, using the form:

INSERT INTO <relation>( <subquery> );


Example: Insert a SubqueryExample: Insert a Subquery

Using Frequents(drinker, bar), enter into the new relation PotBuddies(name) all of Sally’s “potential buddies,” i.e., those drinkers who frequent at least one bar that Sally also frequents.


SolutionSolution

INSERT INTO PotBuddies(SELECT d2.drinker FROM Frequents d1, Frequents d2 WHERE d1.drinker = ‘Sally’ AND

d2.drinker <> ‘Sally’ ANDd1.bar = d2.bar

);

Pairs of Drinkertuples where thefirst is for Sally,the second is forsomeone else,and the bars arethe same.

The otherdrinker


DeletionDeletion

To delete tuples satisfying a condition from some relation:

DELETE FROM <relation>WHERE <condition>;


Example: DeletionExample: Deletion

Delete from Likes(drinker, beer) the fact that Sally likes Bud:

DELETE FROM Likes

WHERE drinker = ‘Sally’ AND

beer = ‘Bud’;


Example: Delete all TuplesExample: Delete all Tuples

Make the relation Likes empty:

DELETE FROM Likes;

Note no WHERE clause needed.Do this command delete the Likes table from database ?


Example: Delete Many TuplesExample: Delete Many Tuples

Delete from Beers(name, manf) all beers for which there is another beer by the same manufacturer.

DELETE FROM Beers bWHERE EXISTS (

SELECT name FROM BeersWHERE manf = b.manf AND

name <> b.name);

Beers with the samemanufacturer anda different namefrom the name ofthe beer representedby tuple b.


UpdatesUpdates

To change certain attributes in certain tuples of a relation:

UPDATE <relation>SET <list of attribute assignments>WHERE <condition on tuples>;


Example: UpdateExample: Update

Change drinker Fred’s phone number to 555-1212:

UPDATE Drinkers

SET phone = ‘555-1212’

WHERE name = ‘Fred’;


Example: Update Several TuplesExample: Update Several Tuples

Make $4 the maximum price for beer:UPDATE Sells

SET price = 4.00

WHERE price > 4.00;


SQL Support for ViewsSQL Support for Views

Views are Part of the SQL DDLAbstracting from Conceptual to External Schema

View Hides the DetailsOne or More Tables in Conceptual Schema May be Combined (in Part) to Form a ViewDon’t Include FKs and Other Internal AttributesTypically, View is Formed by Join of Two or More Relations Utilizing FKs, PKs, etc. As a Result - View is Independent

• Once Formed - View Static/Unchangeable to Insulate User Applications from Conceptual Schema

• Similar in Concept/Intent to “Public Interface”


Features of ViewsView Represents a Restricted Portion (Rows, Columns) of a Relation - External Schema in SQLView is Virtual Table View (Not Stored) and Must be Re-evaluated Every Time - DynamicLike Relation, a View Can Be Deleted at Any TimeAttributes Can Be Renamed in View

Reasons for ViewsSecurity Increasing Application-Data Independence

CREATE VIEW PQ(P#, SUMQTY) AS SELECT P#, SUM(QTY) FROM SP GROUP BY P#;

SQL View DefinitionSQL View Definition


First View: Attribute Names are InheritedCREATE VIEW WORKS_ON1 ASSELECT FNAME, LNAME, PNAME, HOURSFROM EMPLOYEE, PROJECT, WORKS_ONWHERE SSN=ESSN AND PNO=PNUMBER ;

Second View: View attribute names are Aliased via a one-to-one Correspondence with the SELECT-clauseCREATE VIEW DEPT_INFO(DEPT_NAME, NO_OF_EMPS, TOTAL_SAL) AS SELECT DNAME, COUNT (*), SUM (SALARY) FROM DEPARTMENT, EMPLOYEE WHERE DNUMBER=DNO GROUP BY DNAME ;

View Definition in Ongoing ExampleView Definition in Ongoing Example


Queries on ViewsQueries on Views

Retrieve the Last Name and First Name of All Employees Who Work on 'ProjectX'.SELECT PNAME, FNAME, LNAMEFROM WORKS_ON1WHEREPNAME='ProjectX' ;

Without the View WORKS_ON1, this Query Specification Would Require Two Join ConditionsA View Can Be Defined to Simplify Frequently Occurring QueriesDBMS Keeps the View Up-to-date if the Base Tables on Which the View is Defined are ModifiedHence, the View is Realized at the Time we Specify a Query on the View


What is View Update Problem?What is View Update Problem?

Retrieval over View Mirrors a Retrieval over RelationHowever, Update over View may cause Problems!In general, a View Update may Introduce Ambiguity when there is more than one way to Update Underlying RelationsConsider the view PQ Created Below:

Try to Change the Total Quantity SUMQTY of P1 in PQ from “30” to “40”Why Does a view Update Problem occur?

CREATE VIEW PQ(P#, SUMQTY) AS SELECT P#, SUM(QTY) FROM SP GROUP BY P#;

Documents

More on Structured Query Language Indra Budi [email protected]