SQL Sub Queries

A Subquery or Inner query or Nested query is a query within another SQL query and embedded within
the WHERE clause.

A subquery is used to return data that will be used in the main query as a condition to further restrict the data to be retrieved.

Subqueries can be used with the SELECT, INSERT, UPDATE, and DELETE statements along with the operators like =, <, >, >=, <=, IN, BETWEEN etc.

There are a few rules that subqueries must follow:

• Subqueries must be enclosed within parentheses.
• A subquery can have only one column in the SELECT clause, unless multiple columns are in the main query for the subquery to compare its selected columns.
• An ORDER BY cannot be used in a subquery, although the main query can use an ORDER BY. The GROUP BY can be used to perform the same function as the ORDER BY in a subquery.
• Subqueries that return more than one row can only be used with multiple value operators, such as the IN operator.
• The SELECT list cannot include any references to values that evaluate to a BLOB, ARRAY, CLOB, or NCLOB.
• A subquery cannot be immediately enclosed in a set function.
• The BETWEEN operator cannot be used with a subquery; however, the BETWEEN operator can be used within the subquery.

Subqueries with the SELECT Statement

SELECT * FROM CUSTOMERS
WHERE ID IN (SELECT ID
                           FROM CUSTOMERS
                           WHERE SALARY > 4500);

Subqueries with the INSERT Statement

INSERT INTO CUSTOMERS_BKP
SELECT * FROM CUSTOMERSWHERE ID IN (SELECT ID
                           FROM CUSTOMERS) ;

Subqueries with the UPDATE Statement

UPDATE CUSTOMERS
SET SALARY = SALARY * 0.25
WHERE AGE IN (SELECT AGE FROM CUSTOMERS_BKP
                               WHERE AGE >= 27 );

Subqueries with the DELETE Statement

DELETE FROM CUSTOMERS
WHERE AGE IN (SELECT AGE FROM CUSTOMERS_BKP
                               WHERE AGE > 27 )