Can I use MySQL ifnull with a select statement?

Question

I'm trying the following and cannot find out what is wrong:

IF( IFNULL(
      SELECT * FROM vertreter AS ag 
      WHERE ag.iln = param_loginID 
      AND ag.verkaeufer = param_sellerILN
      ),
      UPDATE vertreter AS agUp
      SET agUp.vertreterkennzeichen
      WHERE agUp.iln = param_loginID AND agUp.verkaeufer = param_sellerILN
      ,
      INSERT INTO vertreter AS agIn
          ( agIn.iln, agIn.verkaeufer, agIn.vertreterkennzeichen, agIn.`status` )
      VALUES
          ( param_loginID, param_sellerILN, param_agentID, 'Angefragt' )
      );

Question:
Is this possible at all, to check if a SELECT returns NULL and then do A or B depending?

Answer 1

You need to create unique composite index (iln + verkaeufer).

CREATE UNIQUE INDEX vertreter_iln_verkaeufer ON vertreter (iln, verkaeufer)

http://dev.mysql.com/doc/refman/5.0/en/create-index.html

And then you can do this in one query:

INSERT INTO vertreter
(agIn.iln, agIn.verkaeufer, agIn.vertreterkennzeichen, agIn.`status`)
VALUES (param_loginID, param_sellerILN, param_agentID, 'Angefragt')
ON DUPLICATE KEY UPDATE vertreterkennzeichen = param_agentID

Documentation: http://dev.mysql.com/doc/refman/5.5/en/insert-on-duplicate.html