Beginning SQL

Summary
Queries within Queries
This chapter covered the nesting of a query inside another query, a powerful tool that you can use when
faced with tricky data extraction questions. Specifically, this chapter covered the following topics:
❑ A subquery is just like a normal query, with SELECT clauses, WHERE statements, and GROUP BY
and HAVING clauses.
❑ Subqueries can be included in a SELECT statement’s column listing, or they can be used inside a
WHERE statement to enable results to be filtered.
❑ Aggregate functions are commonly used with subqueries because a subquery must normally
return just one record for each record in the main query.
❑ A subquery can return multiple rows when used with an IN, ANY, ALL, or EXISTS operator.
These operators function as follows:
❑ The IN operator is used to find out whether a value is in one of the values returned by a
subquery.
❑ The ANY operator allows a comparison between a value and any of the values returned
by a subquery. Comparisons are not limited to just equals (=); rather, you can use any
comparison operator, such as greater than (>), less than (