Subqueries

This Oracle tutorial explains how to use Oracle subqueries with syntax and examples.

What is a subquery in Oracle?

In Oracle, a subquery is a query within a query. You can create subqueries within your SQL statements. These subqueries can reside in the WHERE clause, the FROM clause, or the SELECT clause.

WHERE clause

Most often, the subquery will be found in the WHERE clause. These subqueries are also called nested subqueries.

For example:

SELECT *

FROM all_tables tabs

WHERE tabs.table_name IN (SELECT cols.table_name

                          FROM all_tab_columns cols

                          WHERE cols.column_name = 'SUPPLIER_ID');

Limitation: Oracle allows up to 255 levels of subqueries in the WHERE clause.

FROM clause

A subquery can also be found in the FROM clause. These are called inline views.

For example:

SELECT suppliers.name, subquery1.total_amt

FROM suppliers,

 (SELECT supplier_id, SUM(orders.amount) AS total_amt

  FROM orders

  GROUP BY supplier_id) subquery1

WHERE subquery1.supplier_id = suppliers.supplier_id;

In this example, we've created a subquery in the FROM clause as follows:

(SELECT supplier_id, SUM(orders.amount) AS total_amt

 FROM orders

 GROUP BY supplier_id) subquery1

This subquery has been aliased with the name subquery1. This will be the name used to reference this subquery or any of its fields.

Limitations

Oracle allows an unlimited number of subqueries in the FROM clause.

SELECT clause

A subquery can also be found in the SELECT clause.

For example:

SELECT tbls.owner, tbls.table_name,

  (SELECT COUNT(column_name) AS total_columns

   FROM all_tab_columns cols

   WHERE cols.owner = tbls.owner

   AND cols.table_name = tbls.table_name) subquery2

FROM all_tables tbls;

In this example, we've created a subquery in the SELECT clause as follows:

(SELECT COUNT(column_name) AS total_columns

 FROM all_tab_columns cols

 WHERE cols.owner = tbls.owner

 AND cols.table_name = tbls.table_name) subquery2

The subquery has been aliased with the name subquery2. This will be the name used to reference this subquery or any of its fields.

The trick to placing a subquery in the select clause is that the subquery must return a single value. This is why an aggregate function such as SUM function, COUNT function, MIN function, or MAX function is commonly used in the subquery.