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question regarding use of MySQL triggers
DELIMITER |

DROP TRIGGER /*!50032 IF EXISTS */ `trigger_update_results_tb`|

CREATE TRIGGER `trigger_update_results_tb` BEFORE UPDATE ON `results_tb`
    FOR EACH ROW BEGIN
       CASE FLOOR(NEW.score / 10)
        WHEN 0 THEN SET NEW.grade = 'F';
        WHEN 1 THEN SET NEW.grade = 'F';
        WHEN 2 THEN SET NEW.grade = 'F';
        WHEN 3 THEN SET NEW.grade = 'F';
        WHEN 4 THEN SET NEW.grade = 'D';
        WHEN 5 THEN SET NEW.grade = 'C';
        WHEN 6 THEN SET NEW.grade = 'B';
        ELSE SET NEW.grade = 'A';
       END CASE;
    END;
|

DELIMITER ;

SELECT score, grade FROM results_tb WHERE id=1

SELECT r.score, l.grade FROM results_tb r
JOIN lookup l ON r.score = l.score
WHERE r.id=1