CS 122: Lecture 3 Joins (Part 1) Tarik Booker CS 122 California State University, Los Angeles October 7, 2014

What we cover…  Keys  Primary (review), Foreign  Combining Tables  Why, How?  Cartesian Product  Joins  Cross Joins  Named Column Joins  Qualified Table Names  Named Column Join  Equi-Join  Table Aliases  Inner Joins  Natural Joins  Joining More than One Table  Solving Join Problems  Join Expressions

Keys!  Primary Keys  What is the definition of a Primary Key?  Primary Key for GET?  Foreign Keys  Primary Keys in one Table Listed in Another Table  What are the foreign Keys in the Lyric Database?  Why Foreign Keys?  So we can combine tables!

Combining Tables  Why the need to combine tables? Why so many tables anyway?  So far used only one table at a time.  Different tables in our database  One-to-one relationships  Remember the first lecture – parent with one child  One-to-many relationships  One record can be related to many other records  What relates to records in the Titles table?  Many-to-many relationships  Want as little redundancy as possible  What happened if Artists, Titles, and Tracks were instead in one database?  Any redundancies?

Ways to combine tables  Cartesian Product  All possible combinations of rows and columns  Also called “Cross Join”  Joins  Cartesian Product… with Conditions  Different Types of Conditions  Foreign keys  Other (much later)

Cartesian Product  Set of all possible concatenated rows from tables A and B  First component from first table (A)  Second component from second table (B)  First table = “a” rows, Second table = “b” number of rows  Cartesian product = a x b rows  A = 6, B = 4?

Cartesian Product (2)  Genre x Titles?  Genre is a foreign key of Titles  Cartesian Product, so foreign keys don’t matter  Called a CROSS JOIN in SQL  SELECT * FROM Titles CROSS JOIN Genre;  Result?  Lots of info… / Too much info!!

Joins  Subset of the Cartesian Product  Similar (mathematical) operation, but only concatenates if a condition is met  Called the join condition or join predicate  Different types of join conditions  Different types of joins  Table A,B performs Cartesian Product  Only concatenates if join condition is true!

Types of Joins  Many types of Joins  We will cover some this week, some next week  Cross Joins  Named Column Joins  Inner Join  Equi-Join  Natural Joins  Next week  Outer join, LEFT, RIGHT SELF, …

Equi-Join  Joins – take Cartesian Product  Match a column  Remove other rows  Use a comma to select multiple tables  SELECT *  FROM Artists, Titles  WHERE Artists.ArtistID = Titles.ArtistID  Results?  What is the Join Condition?  No join condition = cross join!

Equi-Join (2)  SELECT *  From Artists, Titles  Where Artists.artistID = Titles.artistID;  Why do this?  Just Artist name and titles  SELECT Artistname, Title  FROM Artists, Titles  WHERE Artists.artistID = Titles.artistID;

Equi-Join (3)  Can also use ON keyword instead of WHERE  SELECT attribute_list  FROM A,B  WHERE join_condition;  SELECT attribute_list  FROM A JOIN B  ON join_condition;

Named Column Joins  Tired of typing the same column name twice for every WHERE condition in the Join?  Use Named Column Joins (the JOIN, USING keywords)  SELECT attribute_list  FROM A JOIN B USING(column_name)  Works for joining multiple columns  SELECT attribute_list  FROM A JOIN B USING (name1, name2, …)  SELECT artistname, title, FROM artists JOIN titles USING (artistID);  Don’t forget the parentheses!!

Named Column Joins (2)  Take care! There may be cases where columns are the same name, but different data!  Be alert – you may not get the answer you’re looking for  What about this query? Find all Customers under salesman “Bentley”  SELECT *  FROM Members JOIN SalesPeople  USING (LastName);  Where SalesPeople.Lastname = ‘Bentley’  Why is this wrong?

Named Column Joins (3)  Correct:  Select MEMBERS.Firstname, MEMBERS.Lastname, SalesPeople.Lastname  FrOM MEMBERS JOIN SALESPEOPLE  Using (salesid)  Where SalesPeople.Lastname = 'Bentley';  SalesID is the proper column to join on  Foreign Key of Members  Primary Key of SalesPeople  Many databases have common “address,” “id,” “name” fields  Make sure you’re joining on the correct columns!  When in doubt, don’t use named column join!

Qualified Table Names  Last example:  Select MEMBERS.Firstname, MEMBERS.Lastname, SalesPeople.Lastname  FrOM MEMBERS JOIN SALESPEOPLE  Using (salesid)  Where SalesPeople.Lastname = 'Bentley';  If we SELECT * on the join, we get multiple first and last names  From Members and SalesPeople  Put the table name before the column (and “.”) to specify, or qualify the name.  If there are multiple columns, SQL will call it ambiguous, and cause an error.

Table Aliases  Select MEMBERS.Firstname, MEMBERS.Lastname, SalesPeople.Lastname  FrOM MEMBERS JOIN SALESPEOPLE  Using (salesid)  Where SalesPeople.Lastname = 'Bentley';  Don’t want to type out full table names to qualify?  Use a Table Alias  SELECT M.Firstname, M.Lastname, S.Lastname  FROM Members M, SalesPeople S,  Where M.SalesID = S.SalesID

Table Aliases (2)  Remember Column Aliases?  Can use the AS Keyword to Create Table Aliases  SELECT A.ArtistName, A.ArtistID, T.Title  FROM Artists AS A, Titles AS T  WHERE A.ArtistID = T.ArtistID;

Examples  Is this a lot? Don’t worry!  Ex:  SELECT *  FROM Artists A, Titles T  WHERE A.ArtistID = T.ArtistID;  SELECT M.LastName, S.Studioname  FROM Members M, Studios S  WHERE M.SalesID = S.SalesID  What type of join is this?

Inner Join  Same as Equi-Join, but with different syntax.  SELECT attribute_list  FROM A INNER JOIN B ON join_condition;  Safer than Named Column Join  Doesn’t require the same fields  List the name of each track with its respective title  Answer?  What do we need?  Tables: Tracks, Titles, Keys: TitleID

Inner Join (2)  Answer:  SELECT tr.tracktitle, ti.title  FROM Tracks tr INNER JOIN Titles ti  ON (tr.titleID = ti.titleID);

Natural Joins  Not in textbook  No join condition specified  DBMS determines automatically what to join on  Matches on all fields that have the same names in both tables  Unpredictable – stay away!  SELECT attribute_list  FROM A NATURAL JOIN B;  Will find this in other texts

More on Foreign Keys  Column with values that are Primary Keys of another table  Examples: ArtistID in Titles Table  Other?  Can also protect Data base from anomalies  Try to use joins with foreign keys  Good practice  Most joins use foreign keys

Cross Referencing Tables  Note XrefArtistMembers table  Can allow an individual listed in the members table to be a member of any number of artists.  An artist can also have any number of members  Many-to-many relationship

Joining Multiple (more than 2) Tables  You can chain tables:  SELECT *  FROM A NATURAL JOIN B NATURAL JOIN C;  Why no join condition above?  SELECT *  FROM A JOIN B USING (a1) JOIN C USING(a2);

Joining Multiple Tables (2)  Inner Join:  SELECT *  FROM A INNER JOIN B ON A.n1 = B.n2 INNER JOIN C ON B.n3 = C.n4;  Equi-Join:  SELECT *  FROM A,B,C  WHERE A.n1 = B.n2 AND B.n3 = C.n4;

Joining Multiple Tables (3)  Why?  List all members of the band “The Neurotics’  List all members of “Confused”

Joining Multiple Tables (4)  Select M.Firstname, M.Lastname, A.Artistname, A.ArtistID  FrOM Artists A JOIN XrefArtistsMembers Using (ArtistID) JOIN Members M USING(Memberid)  Where Artistname = 'The Neurotics';

More Examples  Provide the first and last names of all members who have recorded at the studio “MakeTrax”  Solving  What tables are we using?  Start with Members, end with studios  Xrefartistsmembers, Titles  What keys match?

More Examples (2)  Answer:  SELECT DISTINCT m.firstname, m.lastname  FROM Members M JOIN XrefArtistsMembers X USING (memberID)  JOIN Titles t USING (artistID)  JOIN Studios s USING (studioID)  WHERE S.StudioName = ‘maketrax’;

Join Expressions  You can also use joins within parentheses, like mathematical expressions  Such as (x + y) * (a + b)  Ex:  SELECT *  FROM (A JOIN B USING(n1)) JOIN (C JOIN D USING(n2)) USING(n3)  More on this later…