A subquery answers multiple-part questions. For example, to determine who works in Taylor's department, you can first use a subquery to determine the department in which Taylor works. You can then answer the original question with the parent SELECT statement. A subquery in the FROM clause of a SELECT statement is also called an inline view. you can nest any number of subqueries in an inline view. A subquery in the WHERE clause of a SELECT statement is also called a nested subquery. You can nest up to 255 levels of subqueries in the a nested subquery.

A subquery can contain another subquery. Oracle Database imposes no limit on the number of subquery levels in the FROM clause of the top-level query. You can nest up to 255 levels of subqueries in the WHERE clause.

If columns in a subquery have the same name as columns in the containing statement, then you must prefix any reference to the column of the table from the containing statement with the table name or alias. To make your statements easier to read, always qualify the columns in a subquery with the name or alias of the table, view, or materialized view.

Oracle performs a correlated subquery when a nested subquery references a column from a table referred to a parent statement one or more levels above the subquery or nested subquery. The parent statement can be a SELECT, UPDATE, or DELETE statement in which the subquery is nested. A correlated subquery conceptually is evaluated once for each row processed by the parent statement. However, the optimizer may choose to rewrite the query as a join or use some other technique to formulate a query that is semantically equivalent. Oracle resolves unqualified columns in the subquery by looking in the tables named in the subquery and then in the tables named in the parent statement.

A correlated subquery answers a multiple-part question whose answer depends on the value in each row processed by the parent statement. For example, you can use a correlated subquery to determine which employees earn more than the average salaries for their departments. In this case, the correlated subquery specifically computes the average salary for each department.

Use subqueries for the following purposes:

• To define the set of rows to be inserted into the target table of an INSERT or CREATE TABLE statement

• To define the set of rows to be included in a view or materialized view in a CREATE VIEW or CREATE MATERIALIZED VIEW statement

• To define one or more values to be assigned to existing rows in an UPDATE statement

• To provide values for conditions in a WHERE clause, HAVING clause, or START WITH clause of SELECT, UPDATE, and DELETE statements

• To define a table to be operated on by a containing query

  You do this by placing the subquery in the FROM clause of the containing query as you would a table name. You may use subqueries in place of tables in this way as well in INSERT, UPDATE, and DELETE statements.

  Subqueries so used can employ correlation variables, both defined within the subquery itself and those defined in query blocks containing the subquery. Refer to table_collection_expression for more information.

  Scalar subqueries, which return a single column value from a single row, are a valid form of expression. You can use scalar subquery expressions in most of the places where expr is called for in syntax. Refer to "Scalar Subquery Expressions" for more information.