Question

Today, my boss told the below SQL query and went out without explaining it. Its working good.
But i want to know the way how it works.

SELECT NAME 
  FROM PERMISSIONTOKENS 
 WHERE ID IN (SELECT TOKENID 
                FROM ROLETOKENASSOCIATION 
               WHERE ROLEID = '1');

Answer 1

There are two queries here.

The inner query:

SELECT TOKENID FROM ROLETOKENASSOCIATION WHERE ROLEID = '1'

Will get all of the TOKENIDs from ROLETOKENASSOCIATION that have a ROLEID of 1.

The outer query:

SELECT NAME FROM PERMISSIONTOKENS WHERE ID IN(...)

This will get all of the names from PERMISSIONTOKENS that have an ID that was in the result set of the inner query.

You might be able to re-write this with joins instead of using IN, if you don’t like the syntax of IN.