Sams Teach Yourself Transact-SQL in 21 Days, Second Edition

Understanding Subqueries

The first thing that we want to do is explain what a subquery really is. A subquery is a SELECT query that returns a single value and is nested inside a SELECT, INSERT, UPDATE, or DELETE statement or inside another subquery. A subquery can be used anywhere an expression is allowed.

Note

The INSERT, UPDATE, and DELETE statements will be covered in tomorrow's lesson.

A subquery is a normal T-SQL query that is nested inside another query, using parentheses.

You may have a subquery nesting up to 32 levels deep. Thirty-two levels are more than enough for most queries that you will ever work with.

In prior versions of Microsoft SQL Server, you could have a maximum of 16 tables in a SELECT statement. This limit included the ...