A subquery can be part of a search condition.
The search condition is in the form operand operator operand. Either operand can be a subquery. In the following example, the first operand is EMPNO and operator is IN. The search condition can be part of a WHERE or HAVING clause. The clause can include more than one search condition that contains a subquery. A search condition containing a subquery, like any other search condition, can be enclosed in parentheses, can be preceded by the keyword NOT, and can be linked to other search conditions through the keywords AND and OR. For example, the WHERE clause of a query can look something like this:
WHERE (subquery1) = X AND (Y > SOME (subquery2) OR Z = 100)
Subqueries can also appear in the search conditions of other subqueries. Such subqueries are said to be nested at some level of nesting. For example, a subquery within a subquery within an outer-level SELECT is nested at a nesting level of two. SQL allows nesting down to a nesting level of 32.