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

Day 6. Using Subqueries

One of the capabilities of SQL is to nest one SQL query within another. If you want to select data from one table based on a list of values, you could enter the WHERE condition values manually using an IN clause, or you could use a second SQL SELECT statement to get the values. When you use the result set from one query to supply another query's conditions, you are using a subquery. Subqueries enable you to perform in one step what otherwise would require multiple steps. In today's lesson, you will learn

• How to write subqueries

• Why to use subqueries: what they do and why they're useful

• Subqueries returning a single value

• Subqueries returning multiply values

• Using the IN clause with a subquery

• Correlated subqueries

• Using WHERE ...