Basic Syntax

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.

Creating a Basic Subquery

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, Director
FROM Movie, Review
WHERE 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.

Slightly More Complex Subqueries

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 Title
FROM Movie
WHERE 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 mID's whose 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.