Subquery

Oracle/PLSQL: Subqueries

What is a subquery?

A subquery is a query within a query. In Oracle, 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');

Limitations:

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_amtfrom 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) subquery2from 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, COUNT, MIN, or MAX is commonly used in the subquery.

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