1. Using Relational Databases and SQL Steven Emory Department of Computer Science California State University, Los Angeles Lecture 4: Joins Part II

2. Topics for Today Self Joins (Pages 84 - 86)‏ Left and Right Joins (Pages 74 - 80)‏ Mixing Inner and Outer Joins (Pages 81 - 82)‏

3. Self Joins A self join is a table that is joined to itself Typical case is when a foreign key references a primary key in the same table Non-typical case is when you want to find pairs of things from within the same table Useful for modelling hierarchical relationships within a single table Folders: Parent-Child Hierarchy Employees: Employee Hierarchy

4. Typical Self Joins Employee Table: EmployeeID (primary key) FirstName LastName JobTitle Salary SupervisorID (foreign key references EmployeeID)

5. Typical Self Joins #1 List all employees (first and last names) who are supervised by no one. #2 List all employees whose supervisors are supervised by no one. #3 List all employees whose supervisor’s supervisors are supervised by no one.

6. Typical Self Joins Solutions (for #2 and #3): SELECT E.FirstName, E.LastName FROM Employee E JOIN Employee S ON E.SupervisorID = S.EmployeeID WHERE S.SupervisorID IS NULL; SELECT E.FirstName, E.LastName FROM Employee E JOIN Employee S ON E.SupervisorID = S.EmployeeID JOIN Employee SS ON S.SupervisorID = S.EmployeeID WHERE SS.SupervisorID IS NULL;

7. Self Joins in the Movie Database Good news! There are no hierarchical table relationships, so no typical cases There are, however, some non-typical self join possibilities, dealing with pairings

8. Self Joins in the Movie Database Example -- List all pairs of movie-related people who were born in the same city, state/province, and country. Do not pair any people with themselves and do not list duplicate pairs. SELECT P1.FirstName, P1.LastName, '...', P2.FirstName, P2.LastName FROM People P1 JOIN People P2 ON (P1.BirthCity = P2.BirthCity AND P1.BirthStateProvince = P2.BirthStateProvince AND P1.BirthCountry = P2.BirthCountry) WHERE P1.PersonID < P2.PersonID;

9. Self Joins in the Movie Database Alternate (shorter) solution: -- List all pairs of movie-related people who were born in the same city, state/province, and country. Do not pair any people with themselves and do not list duplicate pairs. SELECT P1.FirstName, P1.LastName, '...', P2.FirstName, P2.LastName FROM People P1 JOIN People P2 USING(BirthCity, BirthStateProvince, BirthCountry) WHERE P1.PersonID < P2.PersonID;

10. Outer Joins: The Problem Question: Display a list of movie-related people who are not actors.

11. Outer Joins: The Problem Question: Display a list of movie-related people who are not actors. Let’s try an inner join SELECT FirstName, LastName, MovieID, CharacterName FROM People P JOIN XRefActorsMovies A ON P.PersonID = A.ActorID; Incorrect! An inner join associates people (in P) with movie characters (in A) for which they played.

12. Outer Joins: The Problem Question: Display a list of movie-related people who are not actors. Let’s try negating the inner join condition SELECT FirstName, LastName, MovieID, CharacterName FROM People P JOIN XRefActorsMovies A ON P.PersonID <> A.ActorID; Incorrect as well! An inner join with a negated join condition associates people (in P) with movie characters (in A) for which they did not play.

13. Outer Joins: The Problem To solve the problem, we need to know which PersonIDs in the People table ARE NOT IN the ActorID column of the XRefActorsMovies table?

14. Outer Joins: The Solution Use an outer join SELECT FirstName, LastName FROM People P LEFT JOIN XRefActorsMovies A ON P.PersonID = A.ActorID WHERE ActorID IS NULL; SELECT FirstName, LastName FROM XRefActorsMovies A RIGHT JOIN People P ON P.PersonID = A.ActorID WHERE ActorID IS NULL; Can't use equi-join

15. Outer Join Definition Outer Join = Matching Records (Inner Join) + Zero-Matching Records (Anti Join + NULL) Provides information about records in one table that ARE and ARE NOT in the second table Since we must always have a complete table, NULL values are concatenated to the anti join results

16. Outer Join Visualization

17. Outer Join Types Left Join Every record from the left (first) table will always be listed at least once If a matching record is found in the right (second) table, it is listed normally (same as inner join)‏ If there are no matching records to be found in the right (second) table (zero-matching rows), the record from the left table is still reported, albeit it is associated with NULL values in the right table. Right Join Same as left join, but swapping left and right

18. Outer Join Tips If you need to use an outer join, always use a LEFT JOIN in MySQL LEFT JOIN is same as RIGHT JOIN Difference is do you prefer: Joining left to right? (LEFT JOIN) Joining right to left? (RIGHT JOIN) Mathematical expression and the English grammar read from left to right… therefore I always prefer the LEFT JOIN!

19. Left Join Syntax Two tables SELECT attribute_list FROM table1 LEFT JOIN table2 ON join_condition; More than two tables SELECT attribute_list FROM table1 LEFT JOIN table2 ON join_condition LEFT JOIN table3 ON join_condition... *Note that you may also use JOIN USING syntax as well…

20. Right Join Syntax Two tables SELECT attribute_list FROM table1 RIGHT JOIN table2 ON join_condition; More than two tables SELECT attribute_list FROM table1 RIGHT JOIN table2 ON join_condition RIGHT JOIN table3 ON join_condition... *Note that you may also use JOIN USING syntax as well…

21. More Left/Right Join Syntax NATURAL JOIN syntax SELECT attribute_list FROM table1 NATURAL LEFT JOIN table2; SELECT attribute_list FROM table1 NATURAL RIGHT JOIN table2; JOIN USING syntax SELECT attribute_list FROM table1 LEFT JOIN table2 USING(attribute); SELECT attribute_list FROM table1 RIGHT JOIN table2 USING(attribute);

22. Outer Join Order Join order makes a big difference A LEFT JOIN B is not the same as B LEFT JOIN A The expression “People (People table) who are not actors (XRefActorsMovies table)” does not say the same thing as, “Actors (XRefActorsMovies table) who are not people (People table)” Why not? Only SOME people are actors (A LEFT JOIN B) However, ALL actors are people (B LEFT JOIN A) Thus, you will get different results!

23. Outer Join Order Example Try it and you will see that you do not get the same results!

24. Extracting Data From Outer Joins To extract the “zero-matching” results from an outer join, you must test the primary key (from the opposite side of the outer join) for NULL For example, for A LEFT JOIN B, to extract the records in A that have no matches in B, you must test the primary key in B (the right table) for NULL in the WHERE clause

25. Extracting Data From Outer Joins

27. Mixing Inner and Outer Joins It is OK to mix inner joins with outer joins Once you start an outer join, you usually have to keep doing outer joins (if you join more tables) NULL does not match up with anything Try the following query with mixed join types: -- List the first and last names of ALL movie-related people along with the titles of any movies that they have starred in. If a person has not acted in any movies, just display their first and last name along with a NULL movie title.

28. Mixing Inner and Outer Joins Solution #1: Correct SELECT FirstName, LastName, Title FROM People P LEFT JOIN XRefActorsMovies A ON P.PersonID = A.ActorID LEFT JOIN Movies M ON A.MovieID = M.MovieID;

29. Mixing Inner and Outer Joins Solution #2: Incorrect SELECT FirstName, LastName, Title FROM People P LEFT JOIN XRefActorsMovies A ON P.PersonID = A.ActorID INNER JOIN Movies M ON A.MovieID = M.MovieID;

30. Sample Problems #1 Produce a listing of all genres that are not associated with any movie. #2 Report the first and last names of all movie- related people born in the US who are not producers, directors nor actors.

31. Sample Solutions #1 SELECT G.Genre FROM Genres G LEFT JOIN XRefGenresMovies XGM ON G.Genre = XGM.Genre WHERE MovieID IS NULL;

32. Sample Solutions #2 SELECT PersonID, FirstName, LastName FROM People P LEFT JOIN XRefActorsMovies A ON P.PersonID = A.ActorID LEFT JOIN XRefDirectorsMovies D ON P.PersonID = D.DirectorID LEFT JOIN XRefProducersMovies R ON P.PersonID = R.ProducerID WHERE A.ActorID IS NULL AND D.DirectorID IS NULL AND R.ProducerID IS NULL;