1. SQL queries subqueries and joining

2. RHS – SOC 2 SQL query – beyond one table So far, we have only applied queries to a single table It is possible – and very useful – to make queries applied to several tables Can answer more complex questions… …but queries also tend to become more complex themselves 

3. RHS – SOC 3 SQL query – beyond one table We use a movie information database as example Movie movieid title country prodyear genre oscars Actor actorid name country birth living oscars Casting movieid actorid

4. RHS – SOC 4 SQL query – beyond one table actoridnamecountrybirthlivingoscars 1John WayneUSA04-06-1911No2 2Chow-Yun FatHK12-11-1959Yes0 3Karl SteggerDenmark30-01-1922No0 4Jean RenoFrance10-10-1949Yes0 5Julia RobertsUSA28-09-1964Yes1 6Natalie PortmanIsrael02-07-1980Yes0 7Marc DuretFrance01-08-1966Yes1

5. RHS – SOC 5 SQL query – beyond one table movieidactorid 11 14 15 17 24 26 27 31 32 33 movieidactorid 44 46 52 54 56 65 66 72 74 75

6. RHS – SOC 6 SQL query – subqueries Suppose we wish to answer a (complex) question like this: How many movies are from the country that Marc Duret is from This question cannot be answered by a single query (as we know it…) Information is spread out in multiple tables

7. RHS – SOC 7 SQL query – subqueries SELECT country FROM Actor WHERE name = ’Marc Duret’ SELECT COUNT(title) AS MovieCount FROM Movie WHERE Movie.country = ’France’ Result is ’France’ ’France’ used as input NB!

8. RHS – SOC 8 SQL query – subqueries We can run these two queries ”manually”, or use output from first query as input to second query – not optimal… Better to make first query a subquery to the second query

9. RHS – SOC 9 SQL query – subqueries SELECT COUNT(title) AS MovieCount FROM Movie WHERE country =(SELECT country FROM Actor WHERE name = ’Marc Duret’) Outer query Inner query

10. RHS – SOC 10 SQL query – subqueries Result from inner query (the subquery) is used as input to outer query The inner query produces a result table – just as any other query – which is then used by the outer query Very convenient – but some complications –Name clash –Multiple results

11. RHS – SOC 11 SQL query – subqueries When we query more than one table, field names from different tables might be identical – a name clash We can qualify a field name by prefixing it with the table name –Movie.country (country field in Movie table) –Actor.country (country field in Actor table)

12. RHS – SOC 12 SQL query – subqueries What if a subquery produces more than one result? SELECT country FROM Actor WHERE (oscars > 0) Result: USA France

13. RHS – SOC 13 SQL query – subqueries In such case, we must use the IN keyword SELECT COUNT(title) AS MovieCount FROM Movie WHERE country IN(SELECT country FROM Actor WHERE (oscars > 0)) Try formulating this query in plain English…

14. RHS – SOC 14 Exercise 6 – SQL queries Use the MovieInformation database, defined in exercise 1 Add records to the Actor table, as defined in the presentation Add records to the Casting table, as defined in the presentation With the data in place, run the below queries on the database –SELECT title, prodYear, oscars FROM Movie WHERE oscars > (SELECT SUM(oscars)/5 FROM Movie) –SELECT * FROM Movie WHERE prodYear > (SELECT MAX(prodYear) FROM Movie WHERE (oscars > 0)) Now formulate queries yourself, in order to retrieve the below data: –Get Name and Country for actors from the same country that ’E.T’ was made in –Get Title and Oscars for movies that have won at least as many Oscars as John Wayne has won –Get Name and Birth for actors having won more than twice the average of Oscars won for all actors

15. RHS – SOC 15 SQL query – joining Another approach to multi-table queries is to use join To ”join” tables is to make a kind of ”multiplication” of the tables Simple case: Join without any conditions: SELECT * FROM Movie, Actor

16. RHS – SOC 16 SQL query – joining If the previous query is run against the MovieInformation database, we get a result with 49 rows… 49 = 7 x 7. All combinations of Movie records (7) and Actor records (7) All fields from both tables are included This is rarely what we want…

17. RHS – SOC 17 SQL query – joining In almost all cases, we wish to pick out those rows where certain fields match Example: For each movie, find the title of the movie, and the names of the actors appearing in the movie We already have this information in the Casting table (20 records), but only as identifiers…

18. RHS – SOC 18 SQL query – joining We clearly need information from all three tables in the MovieInformation database, so first attempt at a query could be: SELECT * FROM Movie, Casting, Actor Result: 980 records with a lot of fields…

19. RHS – SOC 19 SQL query – joining First of all, we only need the Title field from the Movie table, and the Name field from the Actor table SELECT Movie.Title, Actor.Name FROM Movie, Casting, Actor Result: 980 records with correct fields

20. RHS – SOC 20 SQL query – joining Next, we only need those records where the relevant information match We are trying to ”substitute” identifiers in the Casting table with real data (titles and names) In order to get the right titles and names, the identifiers for this data must match!

21. RHS – SOC 21 SQL query – joining Match identifier for titles and names: SELECT Movie.Title, Actor.Name FROM Movie, Casting, Actor WHERE ((Movie.MovieId = Casting.MovieId) AND (Actor.ActorId = Casting.ActorId)) Result: 20 records with correct fields!

22. RHS – SOC 22 SQL query – joining This is a quite common ”pattern” for multi- table queries –Tables representing relations only contain identifiers (keys in other tables) –”Real data” is contained in tables representing entities (identifiers are keys) –Obtaining ”real data” for relations requires use of join, with matching of key fields

23. RHS – SOC 23 Exercise 7 – SQL queries Use the MovieInformation database, defined in exercise 1, with data in all three tables With the data in place, run the below queries on the database 1) SELECT Movie.TITLE, COUNT(Actor.ActorId) AS Roles FROM Movie, Actor, Casting WHERE (Actor.COUNTRY = 'France') AND (Casting.ACTORID = Actor.ACTORID) AND (Casting.MOVIEID = Movie.MOVIEID) GROUP BY Movie.TITLE 2) SELECT Movie.Genre, SUM(Actor.Oscars) AS TotalOscars FROM Movie, Actor, Casting WHERE (Actor.Living = 'Yes') AND (Casting.ACTORID = Actor.ACTORID) AND (Casting.MOVIEID = Movie.MOVIEID) GROUP BY Movie.Genre Now formulate queries yourself, in order to retrieve the below data: –Get actor name, country and movie title for those actors appearing in a movie made in their own country –Get actor name and movie title for those actors that have not won an Oscar, but have appeared in Oscar-winning movies