Subqueries

Scalar Subquery

Scalar subqueries are subqueries that return a single value. They can be used anywhere where a regular expression can be used. If a scalar subquery returns more than a single value, the first value returned will be used.

Consider the following table:

Grades

grade	course
7	Math
9	Math
8	CS

CREATE TABLE grades(grade INTEGER, course VARCHAR);
INSERT INTO grades VALUES (7, 'Math'), (9, 'Math'), (8, 'CS');

We can run the following query to obtain the minimum grade:

SELECT MIN(grade) FROM grades;
-- {7}

By using a scalar subquery in the WHERE clause, we can figure out for which course this grade was obtained:

SELECT course FROM grades WHERE grade = (SELECT MIN(grade) FROM grades);
-- {Math}

Exists

The EXISTS operator is used to test for the existence of any row inside the subquery. It returns either true when the subquery returns one or more records, or false otherwise. The EXISTS operator is generally the most useful as a correlated subquery. However, it can be used as an uncorrelated subquery as well.

For example, we can use it to figure out if there are any grades present for a given course:

SELECT EXISTS(SELECT * FROM grades WHERE course='Math');
-- {true}

SELECT EXISTS(SELECT * FROM grades WHERE course='History');
-- false

In Operator

The IN operator checks containment of the left expression inside the result defined by the subquery or the set of expressions on the right hand side (RHS). The IN operator returns true if the expression is present in the RHS, false if the expression is not in the RHS and the RHS has no NULL values, or NULL if the expression is not in the RHS and the RHS has NULL values.

We can use the IN operator in a similar manner as we used the EXISTS operator:

SELECT 'Math' IN (SELECT course FROM grades);
-- true

Correlated Subqueries

All the subqueries presented here so far have been uncorrelated subqueries, where the subqueries themselves are entirely self-contained and can be run without the parent query. There exists a second type of subqueries called correlated subqueries. For correlated subqueries, the subquery uses values from the parent subquery.

Conceptually, the subqueries are run once for every single row in the parent query. Perhaps a simple way of envisioning this is that the correlated subquery is a function that is applied to every row in the source data set.

For example, suppose that we want to find the minimum grade for every course. We could do that as follows:

SELECT *
FROM grades grades_parent
WHERE grade=
    (SELECT MIN(grade)
     FROM grades
     WHERE grades.course=grades_parent.course);
-- {7, Math}, {8, CS}

The subquery uses a column from the parent query (grades_parent.course). Conceptually, we can see the subquery as a function where the correlated column is a parameter to that function:

SELECT MIN(grade) FROM grades WHERE course=?;

Now when we execute this function for each of the rows, we can see that for Math this will return 7, and for CS it will return 8. We then compare it against the grade for that actual row. As a result, the row (Math, 9) will be filtered out, as 9 <> 7.