1. 子查询定义
Any sub-query block in a query statement may be called a subquery; however, we use the term subquery for a sub-query block that appears in the WHERE, SELECT and HAVING clauses. Some Oracle documentation uses the term "nested subquery" for what we refer to as a subquery. A sub-query block that appears in the FROM clause is called a view or derived table.
原文：https://www.cnblogs.com/hopelee/p/13683560.html