The subquery can be a SELECT, INSERT, UPDATE, or DELETE. A subquery can contain other subqueries in the FROM clause, the WHERE clause, or a subselect of a UNION or UNION ALL. A subquery within another subquery is called a child subquery. A subquery that contains another subquery is called a parent subquery. This parent-child relationship can be represented by a tree hierarchy.
If a subquery references at least one column of its parent subquery or of any parent subqueries that are higher up in the tree hierarchy, the subquery is a correlated subquery; otherwise it is a non-correlated subquery. A non-correlated subquery can run at the same time as the highest parent subquery that is also non-correlated. This highest parent subquery is called the "do-at-open parent subquery" in terms of its relationship to the non-correlated subquery. The execution of a correlated subquery is bound to the execution of its parent subquery. Such relationships between the relative executions of parents and children can be represented by separate trees hierarchies in the access plan graph.