how to add trigger in mysql workbench, without UDF, using EER design
A trigger could be like this:

DELIMITER $$

CREATE TRIGGER 
    acts_INSERT_min_salary
BEFORE INSERT ON
    acts
FOR EACH ROW
    BEGIN
        DECLARE msg VARCHAR(255);
        IF NEW.salary &lt; 
            ( SELECT min_salary
              FROM artist
              WHERE artist_id = NEW.artist_artist_id
            )
        THEN 
            SET msg = 'Violation of Minimum Actor Salary.' ;
            SIGNAL SQLSTATE '45000' SET message_text = msg ;
        END IF ;
    END ;

$$
DELIMITER ;


You will need a similar trigger for the UPDATE operation on acts and another one for the UPDATE operation on artist table:

CREATE TRIGGER 
    artist_UPDATE_min_salary
BEFORE UPDATE ON
    artist
FOR EACH ROW
    BEGIN
        DECLARE msg VARCHAR(255);
        IF NEW.min_salary &gt;                             -- this part
            ( SELECT MIN(salary)                        -- is slightly
              FROM acts                                 -- different than
              WHERE artist_artist_id = NEW.artist_id    -- the other 
            )
        THEN 
            SET msg = 'Violation of Minimum Actor Salary.' ;
            SIGNAL SQLSTATE '45000' SET message_text = msg ;
        END IF ;
    END ;