Løsning for dynamiske sætninger i lagrede procedurer kaldet fra triggere

Du kan ikke køre PREPARE /EXECUTE inde fra en TRIGGER , men du kan fra en EVENT (hvis du kører MySQL 5.5 eller nyere).

Her er et eksempel på at køre PREPARE /EXECUTE fra en EVENT :

DROP TABLE IF EXISTS tbl1;
DROP TABLE IF EXISTS tbl2;
DROP TABLE IF EXISTS cmds;
DROP PROCEDURE IF EXISTS proc;
DROP TRIGGER IF EXISTS trig;

CREATE TABLE tbl1 (i INT, v VARCHAR(255));
CREATE TABLE tbl2 (i INT, v VARCHAR(255));

CREATE TABLE cmds (
    id INT UNSIGNED NOT NULL AUTO_INCREMENT,
    done BOOL NOT NULL DEFAULT FALSE,
    cmd TEXT,
    PRIMARY KEY (id),
    INDEX (done, id)
);

DELIMITER //

CREATE PROCEDURE proc()
NOT DETERMINISTIC
MODIFIES SQL DATA
proc: BEGIN
    DECLARE b_not_found     BOOL DEFAULT FALSE;
    DECLARE i_id            INT UNSIGNED;
    DECLARE t_cmd           TEXT;
    DECLARE v_lock_name     VARCHAR(255) DEFAULT 'proc_lock';

    DECLARE cur CURSOR FOR
        SELECT id, cmd FROM cmds WHERE NOT done ORDER BY id;

    DECLARE CONTINUE HANDLER FOR NOT FOUND SET b_not_found = TRUE;

    IF (NOT GET_LOCK(v_lock_name, 0)) THEN
        LEAVE proc;
    END IF;

    OPEN cur;

    loop1: LOOP
        FETCH cur INTO i_id, t_cmd;
        IF b_not_found THEN
            LEAVE loop1;
        END IF;

        SET @cmd = t_cmd;

        PREPARE stmt FROM @cmd;
        EXECUTE stmt;
        DROP PREPARE stmt;

        UPDATE cmds SET done = TRUE WHERE id = i_id;
    END LOOP;

    CLOSE cur;

    DO RELEASE_LOCK(v_lock_name);
END;
//

CREATE TRIGGER trig
    BEFORE INSERT ON tbl1
    FOR EACH ROW
BEGIN
    INSERT INTO cmds SET cmd = 
        CONCAT("INSERT INTO tbl2 SET i = ", -NEW.i, ", v = ", QUOTE(NEW.v));
END;
//

DROP EVENT IF EXISTS evnt //

CREATE EVENT evnt
ON SCHEDULE 
EVERY 1 SECOND
DO
BEGIN
    CALL proc();
END;
//

DELIMITER ;

SET GLOBAL event_scheduler = 1;
 

Kør derefter denne:

INSERT INTO tbl1 VALUES (UNIX_TIMESTAMP(), 'ex 1');
DO SLEEP(2);
INSERT INTO tbl1 VALUES (UNIX_TIMESTAMP(), 'ex 2');
DO SLEEP(1);
SELECT * FROM tbl2;
 

vil producere dette output: