Just like before, our queries will contain a
SELECT clause, a
FROM clause, and a
WHERE clause specifying a condition. However, we now are adding in the ability to nest a
SELECT clause inside the
WHERE clause, thus creating a subquery.
Subqueries can be very powerful when trying to eliminate duplicates, and is often more efficient than using joining relations.
Let's create a query that looks for a movie's ID, title, and director, but only if it has a rating above 4.
We can create a sub-query like so:
SELECT DISTINCT Movie.mID, Title, DirectorFROM Movie, ReviewWHERE Movie.mID in (SELECT mID FROM Review WHERE Rating > 4);
We could easily do this query without implementing a subquery by joining the Movie relation with the Review relation. However, this is just to show how a subquery would be performed.
We would then get the movies: Titanic, and Gravity.
Lets create a query that retrieves the
Title of all movies which have a
Rating less than 3, and have a
mID greater than 103.
Our query would look like this:
SELECT TitleFROM MovieWHERE mID in (SELECT mID FROM Review WHERE Rating < 3)AND mID NOT IN (SELECT mID FROM Review WHERE mID < 103);
Our outside query returns the
Title of all movies whose
mID is in the first subquery, but not in the second subquery. Our first subquery looks for all
Rating is less than 3, and the second subquery looks for all
mIDs that are greater than 103.
The output movies would be: Spiderman, Gravity, and Harry Potter.