SQL queries II. Set operations and joins

Transcription

1 SQL queries II Set operations and joins

2 1. Restrictions on aggregation functions 2. Nulls in aggregates 3. Duplicate elimination in aggregates REFRESHER

3 1. Restriction on SELECT with aggregation If any aggregation is used, then each element of the SELECT column list must be either: 1. Aggregated, or 2. An attribute on the GROUP BY list.

4 Illegal Query Example We might think we could find the shortest movie of Disney as: SELECT title, MIN(length) FROM Movie WHERE studioname = 'Disney'; But this query is illegal in SQL. Because title is neither aggregated nor on the GROUP BY list. We should do instead: SELECT title, length FROM Movie WHERE studioname = 'Disney' AND length = (SELECT MIN(length) FROM Movie WHERE studioname = 'Disney'); 4

5 Or SELECT title, length FROM Movie NATURAL JOIN (SELECT MIN(length) AS length FROM Movie WHERE studioname = 'Disney') WHERE studioname = 'Disney'; 5

6 2. 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. SELECT SUM(networth) FROM moviestar NATURAL FULL OUTER JOIN movieexec; But if there are no non-null values in a column, then the result of the aggregation is NULL. 6

7 Example: Effect of NULL s SELECT count(*) FROM Movie WHERE studioname = 'Disney'; The number of movies from Disney. SELECT count(length) FROM Movie WHERE studioname = 'Disney'; The number of movies from Disney with a known length. 7

8 Useful 3. Duplicate Elimination Find all the different studios producing movies: Movie(title, year, length, incolor, studioname, producerc), SELECT DISTINCT studioname FROM Movie; Without DISTINCT, a studioname would be listed as many times as there were movies from that studio.

9 Eliminating Duplicates in an Aggregation DISTINCT inside an aggregation causes duplicates to be eliminated before the aggregation. Example Find the number of different producers for Disney movies. SELECT COUNT(DISTINCT producerc) FROM Movie WHERE studioname = 'Disney'; This is not the same as: DISTINCT here is useless! Why? SELECT DISTINCT COUNT(producerc) FROM Movie WHERE studioname = 'Disney'; 9

10 Not only in COUNT SELECT AVG(DISTINCT length) FROM Movie WHERE studioname = 'Disney'; This will produce the average of only the distinct values for length: probably an error 10

11 Today Operations on multiple tables Set operations Joins

12 UNION, INERSECT, MINUS (EXCEPT)

13 Bag Semantics and Union, Intersection and Difference Although the SELECT-FROM-WHERE statement uses bag semantics, the default for union, intersection, and difference is set semantics. Motivation? When doing projection in relational algebra, it is harder to eliminate duplicates. When doing intersection or difference, it is most efficient to sort the relations first. At that point you may as well eliminate the duplicates anyway. 13

14 Can we produce bags by set operators? Force the result to be a bag (i.e., don t eliminate duplicates) by ALL, as in UNION ALL... Only UNION ALL supported in ORACLE. 14

15 Union, Intersection, and Difference of Queries If two SQL queries produce relations with the same set of attributes then we can combine the queries using the set operations: UNION, INTERSECT and EXCEPT. 1. Find the names and addresses of all female movie stars who are also movie executives with a net worth over $1,000,000. MovieStar(name, address, gender, birthdate) MovieExec(name, address, cert, networth) (SELECT name, address FROM MovieStar WHERE gender = 'F') INTERSECT (SELECT name, address FROM MovieExec WHERE networth > );

16 Union, Intersection, and Difference of Queries (Continued) 2. Give the names and addresses of movie stars who are not also movie executives. (SELECT name, address FROM MovieStar) MINUS (SELECT name, address FROM MovieExec); Oracle-specific Example. 3. We want all the titles and years of movies that appeared in either the Movie or StarsIn relation. (SELECT title, year FROM Movie) UNION (SELECT title, year FROM StarsIn);

17 JOINS

18 Cartesian product Results from multi-table query that does not have a WHERE clause is a Cartesian product The product results in a huge output which normally is not very useful To avoid a Cartesian product, we use one or more valid join conditions

19 Joins Natural join: R NATURAL JOIN S Example SELECT * FROM MovieStar NATURAL JOIN MovieExec; Safer: SELECT * FROM MovieStar JOIN MovieExec USING(name); Theta join: R JOIN S ON <condition> Example SELECT * FROM MovieStar JOIN MovieExec ON moviestar.name = movieexec.name;

20 Multiple joins are required to collect information from multiple tables Movie ( title, year, length, incolor, studioname, producerc) MovieStar (name, address, gender, birthdate) StarsIn (movietitle, movieyear, starname) MovieExec (name, address, cert, networth) Studio (studioname, presc); Select m.title as MovieTitle, m.year AS MovieYear, p.name AS producer, s.starname AS star From Movie m, StarsIn s, MovieExec p Where m.title = s.title AND m.year = s.year AND p.cert = m.producerc

21 Outerjoin. Motivation Suppose we join R S. A tuple of R which doesn't join with any tuple of S is said to be dangling. Similarly for a tuple of S. Problem: We loose dangling tuples. Outerjoin Preserves dangling tuples by padding them with a NULL in the result. 21

22 Types of outer join R S Full outerjoin. Pad dangling tuples from both tables. R L S Left outerjoin: Only pad dangling tuples from the left table. R R S Right outerjoin: Only pad dangling tuples from the right table. 22

23 Full Outer Join: R S R = A B S = B C (1,2) joins with (2,3), but the other two tuples are dangling. R S = A B C NULL NULL

24 Left Outer Join: R L S R = A B S = B C The result of a left outer join (or simply left join) for table A and B always contains all records of the "left" table (A), even if the join-condition does not find any matching record in the "right" table (B). R L S = A B C NULL 24

25 Right Outer Join: R R S R = A B S = B C A right outer join (or right join) closely resembles a left outer join, except with the treatment of the tables reversed. Every row from the "right" table (B) will appear in the joined table at least once. R R S = A B C NULL

26 Example: Outerjoin R = A B S = B C (1,2) joins with (2,3), but the other two tuples are dangling. R S = A B C NULL NULL 6 7 SELECT * FROM R FULL OUTER JOIN S USING(B);

27 SELECT * Outerjoin examples FROM moviestar NATURAL FULL OUTER JOIN movieexec; SELECT * FROM moviestar NATURAL LEFT OUTER JOIN movieexec; SELECT * FROM moviestar NATURAL RIGHT OUTER JOIN movieexec; LEFT = pad dangling tuples of moviestar only. RIGHT = pad dangling tuples of movieexec only. FULL = pad both.

28 Joins with grouping operators From Movie and MovieExec, find the producer s total length of film produced: SELECT name, SUM(length) FROM Movie, MovieExec WHERE producerc = cert GROUP BY name; Compute those tuples first, then group by name. 28

29 SUBQUERIES OR JOINS?

30 Example 1 Find the name of the producer of Star Wars. Movie(title, year, length, incolor, studioname, producerc) MovieExec(name, address, cert, networth) We can do: SELECT name FROM Movie, MovieExec WHERE title ='Star Wars' AND producerc =cert; Or we can have a subquery: SELECT name FROM MovieExec WHERE cert =(SELECT producerc FROM Movie WHERE title = 'Star Wars'); If we can deduce that there will be only a single value produced by the subquery, then we can use this expression, surrounded by parentheses, as if it were a constant. 30

31 Example 2. Correlated Subqueries Suppose StarsIn relation has an additional attribute salary StarsIn(movieTitle, movieyear, starname, salary) Now, find the stars who were paid for some movie more than the average salary for that movie. SELECT starname, movietitle, movieyear FROM StarsIn X WHERE salary > (SELECT AVG(salary) FROM StarsIn WHERE movietitle = X.movieTitle AND movieyear=x.movieyear); Remarks 1. Outer query cannot reference any columns in the subquery. 2. Subquery references the tuple in the outer query. 3. Value of the tuple changes by row of the outer query, so the database must rerun the subquery for each row comparison. 31

32 Another Solution (Subquery in FROM) SELECT X.starName, X.movieTitle, X.movieYear FROM StarsIn X, (SELECT movietitle, movieyear, AVG(salary) AS avgsalary FROM StarsIn GROUP BY movietitle, movieyear) Y WHERE X.salary>Y.avgSalary AND X.movieTitle=Y.movieTitle AND X.movieYear=Y.movieYear; 32

33 Example 3. Join multiple tables Give all the producers of movies in which Julia Roberts stars. SELECT name FROM MovieExec, Movie, StarsIn WHERE cert = producerc AND title = movietitle AND year = movie Year AND starname = 'Julia Roberts'; 33

34 Using subqueries Give all the producers of movies in which Julia Roberts stars. SELECT name FROM MovieExec WHERE cert IN (SELECT producerc FROM Movie WHERE (title, year) IN (SELECT movietitle, movieyear FROM StarsIn WHERE starname = 'Julia Roberts')); 34

35 Movies EXERCISES

36 Movie ( title, year, length, incolor, studioname, producerc) MovieStar (name, address, gender, birthdate) StarsIn (movietitle, movieyear, starname) MovieExec (name, address, cert, networth) 1. For each movie s title produce the name of this movie s producer 2. Find the names of all producers who did NOT produce Star wars

37 3. Find titles of movies that were produced by the same producer but in different studios: Disney and MGM

38 4. Find all movie titles for which there is no producer entry in MovieExec table

39 5. Find the distinct names of producers of movies where Harrison Ford starred without using distinct

40 6. Find the names of all stars which starred in at least 3 movies (according to our database)

41 1. For each movie s title produce the name of this movie s producer SELECT title AS movie, name AS producer FROM Movie, MovieExec WHERE Movie.producerC = MovieExec.cert; 2. Find the names of all producers who did NOT produce Star wars SELECT name FROM Movie, MovieExec WHERE title = 'Star Wars' AND producerc <> cert;

42 3. Find titles of movies that were produced by the same producer but in different studios: Disney and MGM SELECT DisneyMovie.title, MGMmovie.title FROM (SELECT * FROM Movie WHERE studioname = 'Disney') DisneyMovie, (SELECT * FROM Movie WHERE studioname = 'MGM') MGMmovie WHERE DisneyMovie.producerC = MGMmovie.producerC;

43 4. Find all movie titles for which there is no producer entry in MovieExec table SELECT title FROM Movie WHERE producerc <> ALL (SELECT cert FROM MovieExec); --- <> ALL is the same as NOT IN TITLE Pretty Woman The Man Who Wasn't There Logan's run Star Trek Star Trek: Nemesis The Usual Suspects 6 rows selected.

44 5. Find the distinct names of producers of movies where Harrison Ford starred without using distinct SELECT name FROM MovieExec WHERE cert IN (SELECT producerc FROM Movie WHERE (title, year) IN (SELECT MovieTitle, MovieYear FROM StarsIn WHERE StarName = 'Harrison Ford')); NAME George Lucas

45 6. Find the names of all stars which starred in at least 3 movies (according to our database) SELECT StarName FROM StarsIn Star1 WHERE 2 < (SELECT count(starname) FROM StarsIn Star2 WHERE Star2.MovieTitle = Star1.MovieTitle AND Star2.MovieYear = Star1.MovieYear); -- Correlated subqueries: STARNAME Carrie Fisher Mark Hamill Harrison Ford

46 Bonus: TOP-N analysis Top-N queries are used to sort rows in a table and then to find the first-m largest (smallest) values The sorted rows produced with ORDER BY clause are numbered with pseudocolumn ROWNUM You can display the required number of rows based on ROWNUM and using < or <= operators

47 Example: TOP-4 largest rooms SELECT rownum, Building, RoomNo, Capacity FROM (SELECT Building, RoomNo, Capacity FROM location ORDER BY Capacity DESC) WHERE rownum <=4;

48 Example: TOP-3 lowest salaries SELECT rownum, Lname, Fname, Salary FROM (SELECT Lname, Fname, Salary FROM employee ORDER BY Salary) WHERE rownum <=3;

49 Example: correlated subquery The inner (nested) query can reference columns from the outer query The inner query is executed once for each row of the outer query, while in non-correlated subqueries the inner query was executed only once SELECT EmpID, Salary, DeptID FROM employee outer WHERE Salary = (SELECT MAX (Salary) FROM employee WHERE DeptID = outer.deptid GROUP BY DeptID) Employees with maximum salary in their own department

50 Example: correlated subquery with EXISTS EXISTS (NOT EXISTS) are used with correlated subqueries The EXISTS operator checks if the inner query returns at least one row, and it returns TRUE or FALSE SELECT FacultyID, Name FROM faculty outer WHERE EXISTS (SELECT 1 FROM student WHERE FacultyID = outer. FacultyID); All Faculty members who are in the Student table