1. Databases : SQL Multi-Relations 2007, Fall Pusan National University Ki-Joune Li These slides are made from the materials that Prof. Jeffrey D. Ullman distributes via his course web page (http://infolab.stanford.edu/~ullman/dscb/gslides.html)http://infolab.stanford.edu/~ullman/dscb/gslides.html

2. STEMPNU 2 Multi-Relation Queries More than one relation. Several relations in the FROM clause. Distinguish attributes of the same name  “. ”  Example: Using Likes(drinker, beer) and Frequents(drinker, bar), find the beers liked by at least one person who frequents Joe ’ s Bar. SELECT beer FROM Likes, Frequents WHERE bar = ‘Joe’’s Bar’ AND Frequents.drinker = Likes.drinker;

3. STEMPNU 3 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.

4. STEMPNU 4 Operational Semantics Imagine one tuple-variable for each relation in the FROM clause.  These tuple-variables visit each combination of tuples, one from each relation. If the tuple-variables are pointing to tuples that satisfy the WHERE clause, send these tuples to the SELECT clause. Nested Algorithm For each tuple r1 in R1 For each tuple r2 in R2 if the condition(r1, r2) in WHERE clause is true then print the attributes in SELECT clause Problem of this nested Algorithm SELECT R.A FROM R, S, S WHERE R.A = S.A OR R.A = T.A when T is empty set

5. STEMPNU 5 Example drinker bardrinker beer b1b2 Sally Bud Sally Joe’s Likes Frequents to output check these are equal check for Joe

6. STEMPNU 6 Explicit Tuple-Variables Sometimes, a query needs to use two copies of the same relation. Distinguish copies by tuple-variables in FROM clause. Example: From Beers(name, manf),  find all pairs of beers by the same manf. 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;

7. STEMPNU 7 Union, Intersection, Difference Example  MovieStar(name, address, gender, birthdate) MovieExec(name, address, cert#, netWorth)  Find names and addresses of all female movie stars who are also movie executives with a net worth over $10M (SELECT name, address FROM MovieStar WHERE gender=‘F’) INTERSECT (SELECT name, address FROM MovieExec WHERE netWorth > 10000000)

8. STEMPNU 8 Subqueries Parenthesized SFW statement (subquery) can be used  as a value: returns ONE tuple  as tuples: returns a set of tuples  related with relations: returns Boolean value  in FROM clause

9. STEMPNU 9 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.  Also typically, a single tuple is guaranteed by “ keyness ” of attributes.  A run-time error occurs if there is no tuple or more than one tuple.

10. STEMPNU 10 Example From Sells(bar, beer, price), find the bars that serve Miller for the same price Joe charges for Miller. Two queries would surely work: 1. Find the price Joe charges for Miller. 2. Find the bars that serve Miller at that price.

11. STEMPNU 11 Query + Subquery Solution SELECT bar FROM Sells WHERE beer = ‘Miller’ AND price = ( SELECT price FROM Sells WHERE bar = ‘Joe’’s Bar’ AND beer = ‘Miller’); The price at which Joe sells Bud SELECT s1.bar FROM Sells s1,Sells s2 WHERE s1.beer = ‘Miller’ AND s1.price=s2.price AND s2.bar=‘Joe’’s Bar’ AND s2.beer = ‘Miller’;

12. STEMPNU 12 Subqueries That Return More than one Tuple If a subquery may produce more than one tuple,  then the subquery can be used as a set of values  Set operator are used: IN and NOT IN operators IN operator  IN is true if and only if the tuple is a member of the relation.  NOT IN means the opposite.  IN-expressions can appear in WHERE clauses.  The is often a subquery.

13. STEMPNU 13 Example From Beers(name, manf) and Likes(drinker, beer), find the name and manufacturer of each beer that Fred likes. The set of beers Fred likes SELECT beer.* FROM Beers beer, Likes like WHERE beer.name=Likes.beer AND Likes.drinker=‘Fred’; SELECT * FROM Beers WHERE name IN (SELECT beer FROM Likes WHERE drinker = ‘Fred’);

14. STEMPNU 14 The Exists Operator EXISTS( ) is true if and only if the is not empty. Being a boolean-valued operator, EXISTS can appear in WHERE clauses. Example:  From Beers(name, manf),  find those beers that are the unique beer by their manufacturer.

15. STEMPNU 15 Example Query with EXISTS SELECT name FROM Beers b1 WHERE NOT EXISTS( SELECT * FROM Beers WHERE manf = b1.manf AND name <> b1.name); Set of beers with the same manf as b1, but not the same beer Notice scope rule: manf refers to closest nested FROM with a relation having that attribute.

16. STEMPNU 16 The Operator ANY x = ANY( ) is a boolean condition meaning that x equals at least one tuple in the relation. Similarly, = can be replaced by any of the comparison operators. Example: x >= ANY( ) means x is not smaller than all tuples in the relation.  Note tuples must have one component only.

17. STEMPNU 17 The Operator ALL Similarly, x <> ALL( ) 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( ) means there is no tuple larger than x in the relation.

18. STEMPNU 18 Example From Sells(bar, beer, price),  find the beer(s) sold for the highest price. price from the outer Sells must not be less than any price. SELECT beer FROM Sells WHERE price >= ALL( SELECT price FROM Sells);

19. STEMPNU 19 Example From Movie(title, year, length, inColor, studioName, producerC#),  find movie titles used for more than one film year of movie production with the same title. SELECT title FROM Movie Old WHERE year < ANY( SELECT year FROM Movie title=Old.title);

20. STEMPNU 20 Subqueries in FROM Clause Subqueries can be used in FROM Clause  StarsIn(movieTitle, movieYear, starName) MovieExec(name, address, cert#, netWorth) Movie(title, year, length, inColor, studioName, producerC#)  Find producer the names of Harrison Ford’s movies Set of producerC# of Harrison Ford’s movies SELECT name FROM MovieExec, (SELECT producerC# FROM Movie, StarsIN WHERE title=movieTitle AND year=movieYear AND starName=‘Harrison Ford’ ) Prod WHERE cert#=Prod.producerC#; Foreign Key

21. STEMPNU 21 Example  MovieStar(name, address, gender, birthdate) MovieExec(name, address, cert#, netWorth) CROSS JOIN: Cartisan Product  Movie CROSS JOIN StarsIN JOIN: Theta Join  Movie JOIN StarsIN on title=movieTitle AND year movieYear NATURAL JOIN: Natural Join  MovieStar NATURAL JOIN MovieExec JOIN Condition of Theta Join Join on (name, address)