Day 5 - More Complexity With Queries Explanation of JOIN & Examples Explanation of JOIN & Examples Explanation & Examples of Aggregation Explanation & Examples of Aggregation

CSE 498Day 5 - More Complexity With Queries2 / 12 What is a join? SQL allows you to cross reference information in tables. SQL allows you to cross reference information in tables. There are two methods to do this: There are two methods to do this: –Compare & Contrast: Compare values in a given table against another and get a matching pair result table from the data. –JOIN: Join the two tables into a temporary table based on some rules.

CSE 498Day 5 - More Complexity With Queries3 / 12 More Complex Movie DB A complex version of the Movie DB will be used for this set of slides. Here is the DB schema… A complex version of the Movie DB will be used for this set of slides. Here is the DB schema… Inventory (TapeID,MovieID) Inventory (TapeID,MovieID) Movies (MovieID, MovieName) Movies (MovieID, MovieName) Suppliers (SupplierID, SupplierName) Suppliers (SupplierID, SupplierName) MovieSupplier (SupplierID, MovieID, Price) MovieSupplier (SupplierID, MovieID, Price) Orders (OrderID, SupplierID, MovieID, Copies) Orders (OrderID, SupplierID, MovieID, Copies) This database is for a movie company. This database keeps track of the possible movies available on video. This database also keeps track of different suppliers in the company and the videos they have ordered. This database is for a movie company. This database keeps track of the possible movies available on video. This database also keeps track of different suppliers in the company and the videos they have ordered.

CSE 498Day 5 - More Complexity With Queries4 / 12 Compare & Contrast Tables This is done by putting a statement in the WHERE clause of a query. This is done by putting a statement in the WHERE clause of a query. This will simply compare the two tables and add the matching pairs to the result set. This will simply compare the two tables and add the matching pairs to the result set. Example: Give me the list of all the movies in the inventory. Example: Give me the list of all the movies in the inventory. SELECT DISTINCT M.MovieName SELECT DISTINCT M.MovieName FROM Movies M, Inventory I FROM Movies M, Inventory I WHERE M.MovieID = I.MovieID WHERE M.MovieID = I.MovieID

CSE 498Day 5 - More Complexity With Queries5 / 12 What about JOIN? There are three ways to use JOIN… There are three ways to use JOIN… –INNER JOIN –LEFT JOIN –RIGHT JOIN INNER JOIN INNER JOIN –Does the same thing as compare & contrast. –Syntax : INNER JOIN ON –Syntax : INNER JOIN ON –Best described as pulling together two tables that sit at the same level.

CSE 498Day 5 - More Complexity With Queries6 / 12 LEFT JOIN / RIGHT JOIN Idea of These Joins Idea of These Joins –The result table comes from two tables...and upper table and a lower table. Theory Behind Upper & Lower Tables Theory Behind Upper & Lower Tables –All the records of the upper table are included in the result table. –All matching instances with the lower table are included in the result table with the pair from the higher table. –All records in the upper table that have no matching instance in the lower table have their lower table values filled in with NULL.

CSE 498Day 5 - More Complexity With Queries7 / 12 LEFT / RIGHT cont. Syntax is similar to an INNER JOIN... Syntax is similar to an INNER JOIN... –LEFT JOIN Syntax LEFT JOIN ON LEFT JOIN ON –RIGHT JOIN Syntax RIGHT JOIN ON RIGHT JOIN ON Useful if you’re looking for non-matching pairs... Useful if you’re looking for non-matching pairs... –IS NULL This can be used in the WHERE clause to check if an attribute to some record has a null value. This can be used in the WHERE clause to check if an attribute to some record has a null value. –IS NOT NULL This also can be used in the WHERE clause... This also can be used in the WHERE clause...

CSE 498Day 5 - More Complexity With Queries8 / 12 Example data and Question Given... What movie titles are not in the movie inventory? First Query Try... SELECT * FROM Movie M LEFT JOIN Inventory I ON M.MovieID = I.MovieID; Gives the result... Inventory TapeIDMovieID Movies MovieIDMovieName 1Star Wars 2Empire Strikes Back 3Return of the Jedi MovieIDMovieNameTapeID 1Star Wars1 2Empire Strikes BackNULL 3Return of the Jedi2 3Return of the Jedi3

CSE 498Day 5 - More Complexity With Queries9 / 12 Answer & A New Question... What movie titles are not in the movie inventory? Second Query Try...only look for the null valued answers... SELECT * FROM Movie M LEFT JOIN Inventory I ON M.MovieID = I.MovieID WHERE I.TapeID IS NULL; Gives the result... Thus, just change the SELECT statement to what you want and you’re done! Try this query... Question: What are the names of the movies in the movie inventory? (Hint: Do I have to watch for duplicates?) MovieIDMovieNameTapeID 2Empire Strikes BackNULL

CSE 498Day 5 - More Complexity With Queries10 / 12 What is aggregation? Aggregation is the ability to do simple mathematical functions on groups of data in the result table. Aggregation is the ability to do simple mathematical functions on groups of data in the result table. Methods provided in MySQL... Methods provided in MySQL... –SUM : Adds up any grouped values –MIN : Finds the smallest value in the set given –MAX : Finds the largest value in the set given –AVG : Finds the mean value of the set given –COUNT : Counts up the number of records in the set given To understand how to use aggregation, you have to understand how a query is executed... To understand how to use aggregation, you have to understand how a query is executed...

CSE 498Day 5 - More Complexity With Queries11 / 12 Understanding a Query... Query Format Query Format SELECT SELECT FROM FROM WHERE WHERE GROUP BY GROUP BY HAVING HAVING Execution Order of the Query... Execution Order of the Query... –1st: It finds the tables/joined tables it needs in the FROM clause. –2nd: It evaluates row actions defined in the WHERE clause to produce a temporary result table. –3rd: It uses the GROUP BY and HAVING definitions to know what further actions to take. –4th: It uses the SELECT clause to know what rows to show.

CSE 498Day 5 - More Complexity With Queries12 / 12 Aggregation cont. This can be used in the SELECT or HAVING clause... This can be used in the SELECT or HAVING clause... Examples... Examples... Question: What is the average price of a movie? SELECT AVG(Price) AS Average_Price FROM MovieSupplier; Question: What movie suppliers have a movie that costs less than $9? SELECT DISTINCT S.SupplierName FROM MovieSupplier MS INNER JOIN Supplier S ON S.SupplierID = MS.SupplierID GROUP BY MS.SupplierName HAVING MIN(MS.Price) < 9.00;