CREATE TRIGGER triggername
ON tablename
AFTER INSERT, UPDATE
AS
  BEGIN
    IF EXISTS(SELECT 1
              FROM   INSERTED
              WHERE  ( [field1] = 1  --this column must actually exist in the table 
                   AND [field2] IS NULL )) --this column must actually exist in the table 
      BEGIN
        ROLLBACK TRANSACTION
        RAISERROR ('Text of error: Field2 cannot be null when Field1 has a value of one.',16,1)
      END
  END
GO