CREATE TRIGGER MyTrigger
    ON dbo.EmploymentTable
    AFTER INSERT, UPDATE, DELETE
AS

UPDATE E
SET HasMultEmployer = X.T_HasMultEmployer 
FROM EmployeeTable AS E
     JOIN
     (
         SELECT EmployeeId, [T_HasMultEmployer] = CASE WHEN T_EmploymentCount <= 1 THEN 0 ELSE 1 END
         FROM
         (
             SELECT EmployeeId, [T_EmploymentCount] = COUNT(*)
             FROM   EmploymentTable
             WHERE  EmployeeId IN
                    (
                       SELECT EmployeeId
                       FROM inserted
                       UNION
                       SELECT EmployeeId
                       FROM deleted
                    )
             GROUP BY EmployeeId
         ) AS Y
     ) AS X
         ON X.EmployeeId = E.EmployeeId
WHERE (X.T_HasMultEmployer <> E.HasMultEmployer OR E.HasMultEmployer IS NULL)