Chapter 4
Subqueries
SQL supports writing queries within queries, or nesting queries. The outermost query is a query whose result set is returned to the caller and is known as the outer query. The inner query is a query whose result set is used by the outer query and is known as a subquery. The inner query acts in place of an expression that is based on constants or variables and is evaluated at run time. Unlike the results of expressions that use constants, the result of a subquery can change, because of changes in the queried tables. When you use subqueries, you avoid the need for separate steps in your solutions that store intermediate query results in variables.
A subquery can be either self-contained or correlated. A self-contained subquery ...
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