database - Can I use EXISTS keyword in sql query for assignment of result -

Can I specify a value with the query EXISTS ? Currently it passes me to incorrect syntax '=' . But when I use this query, then only keeps going as expected.

  if the current (SELECT @ PK_LOGIN_ID = dbo.M_LOGIN WHERE LOGIN_NAME = the PK_LOGIN_ID @ LOGIN_NAME and PASSWORD = @ PASSWORD and FK_ROLE_ID = @ FK_ROLE_ID) returns @PK_LOGIN_ID ELSE back 0  

  returns Koylas ((SELECT TOP 1 PK_LOGIN_ID) WHERE the Dbo.M_LOGIN LOGIN_NAME = @LOGIN_NAME and PASSWORD = @PASSWORD and FK_ROLE_ID = @FK_ROLE_ID), 0)