SUB QUERIES

A SUB QUERY is a query within a query which can return one or more rows

SELECT * FROM SALES
WHERE CUSTOMER_ID =
(SELECT CUSTOMER_ID FROM CUSTOMER WHERE LAST_NAME = 'JOSEPH'

A SUBQUERY executes first before the main query

SELECT * FROM SALES
WHERE CUSTOMER_ID IN  
(SELECT CUSTOMER_ID FROM CUSTOMER WHERE REGION ='SOUTH')

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