UNION inside IF EXISTS statement not working

Question

SELECT A, B, C FROM TUser

UNION

IF EXISTS(SELECT dataUserId FROM TUserData WHERE DataId = @dataId AND UserId = @userId)
  BEGIN
    SELECT @dataUserId  = dataUserId FROM TUserData WHERE DataId = @dataId AND UserId = @userId

    SELECT A, B, C FROM TUser WHERE UserId = dataUserId 
  END

Answer 1

You can transform it like that:

SELECT @dataUserId  = dataUserId FROM TUserData
WHERE DataId = @dataId AND UserId = @userId

IF (@dataUserId IS NOT NULL)
  BEGIN
    SELECT A, B, C FROM TUser
    UNION
    SELECT A, B, C FROM TUser WHERE UserId = @dataUserId 
  END
ELSE
  SELECT A, B, C FROM TUser

Answer 2

That is invalid SQL. It appears that you are trying to write something like a function/stored-procedure.

This is how UNION works (http://www.w3schools.com/SQL/sql_union.asp):

SELECT column_name(s) FROM table_name1
UNION
SELECT column_name(s) FROM table_name2

Both SELECT statements must have the same number of columns. The columns must also have similar data types. Also, the columns in each SELECT statement must be in the same order.