What Is a Subquery?
Simply put, a subquery is a SELECT expression that you embed inside one of the clauses of a SELECT statement to form your final query statement. In this chapter, we’ll define more formally a subquery and show how to use it other than in the FROM clause.
The SQL Standard defines three types of subqueries:
Row subquery—an embedded SELECT expression that returns more than one column and no more than one row
Table subquery—an embedded SELECT expression that returns one or more columns and zero to many rows
Scalar subquery—an embedded SELECT expression that returns only one column and no more than one row
Row Subqueries
You’ve already created queries that embed a SELECT statement in a FROM clause to let you filter rows before joining ...
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