EXISTS Operator in SQL

EXISTS Operator in SQL

The EXISTS operator is used to check the existence, it is a logical operator that returns true or false only.

We can use them within IF or Sub Queries.

USING WITH IF :

IF EXISTS (

              SELECT [ COLUMN_NAME ]

              FROM [ TABLE_NAME ]

              WHERE [ COLUMN_NAME ] IS NOT NULL

              )

BEGIN

       --QUERY TO DO ( SELECT [ID] FROM [TABLE_NAME] WHERE [ COLUMN_NAME ] IS NOT NULL)

END

USING WITH SUB QUERY :

SELECT [ID]

FROM [TABLE_NAME]

WHERE EXISTS (

              SELECT [ COLUMN_NAME ]

              FROM [ TABLE_NAME ]

              WHERE [ COLUMN_NAME ] IS NOT NULL

              )

Note: when we are using EXISTS operator in sub query, it will return true even when the sub query return NULL, as showing below in example :

SELECT [ COLUMN_NAME ]

FROM [ TABLE_NAME ]

WHERE EXISTS (

              SELECT NULL

              )

In the above query EXISTS will return true and query will return all data without any condition or filter.