簡單觸發器投擲的錯誤，並停止執行

Question

我有這個觸發器在MySQL

DROP TRIGGER IF EXISTS BeforeGivingRight; 
DELIMITER $$ 
CREATE TRIGGER BeforeGivingRight 
BEFORE INSERT ON right_code 
FOR EACH ROW 
BEGIN 
    DECLARE num_rows INT NULL; 
    SELECT count(right_code) FROM valid_rights WHERE right_code = 
              new.user_right.right INTO num_rows; 

    IF num_rows = 0 
    THEN 
     set msg = concat('Error: That right is not allowed!', 
             cast(new.user_right.right as char)); 
     signal sqlstate '45000' set message_text = msg; 
    END IF; 
END $$ 
DELIMITER ; 

及其給我這個錯誤

#1064 - You have an error in your SQL syntax; check the manual that corresponds 
    to your MySQL server version for the right syntax to use near 'NULL; SELECT 
    count(right_code) FROM valid_rights WHERE right_code = new.user_' at line 5 

這裏有什麼問題？我想要做的是，在插入一些權利（例如201）之前，使用觸發器檢查valid_rights是否存在。

EDIT（回答）

所以，當你想要的trows異常觸發您使用此：

DROP TRIGGER IF EXISTS BeforeGivingRight; 
DELIMITER $$ 
CREATE TRIGGER BeforeGivingRight 
BEFORE INSERT ON user_rights 
FOR EACH ROW 
BEGIN 
    DECLARE num_rows INT DEFAULT NULL; 
    DECLARE msg VARCHAR(32) DEFAULT ""; 
    SELECT count(valid_rights.right_code) FROM valid_rights WHERE right_code = new.right INTO num_rows; 

    IF num_rows = 0 
    THEN 
     set msg = concat('Error: That right is not allowed!', cast(new.right as char)); 
     signal sqlstate '45000' set message_text = msg; 
    END IF; 
END $$ 
DELIMITER ; 

Answer 1

你錯過了關鍵字DEFAULT。使用0而不是NULL。

DECLARE total_sale INT DEFAULT 0; 

See HERE: Variables in Stored Procedures

Answer 2

作爲一個解決方法：如果表是InnoDB的，那麼你可以使用外鍵約束不存在的值。只需在user_rights表中添加外鍵，服務器就會引發錯誤。

例如 -

ALTER TABLE right_code 
    ADD CONSTRAINT FK_right_code FOREIGN KEY (`right`) 
    REFERENCES valid_rights(right_code) ON DELETE RESTRICT ON UPDATE RESTRICT; 

此外，使字段valid_rights。如果不是，則爲right_code。

Answer 3

所以，當你想要的trows異常觸發您使用此：

DROP TRIGGER IF EXISTS BeforeGivingRight; 
DELIMITER $$ 
CREATE TRIGGER BeforeGivingRight 
BEFORE INSERT ON user_rights 
FOR EACH ROW 
BEGIN 
    DECLARE num_rows INT DEFAULT NULL; 
    DECLARE msg VARCHAR(32) DEFAULT ""; 
    SELECT count(valid_rights.right_code) FROM valid_rights WHERE right_code = new.right INTO num_rows; 

    IF num_rows = 0 
    THEN 
     set msg = concat('Error: That right is not allowed!', cast(new.right as char)); 
     signal sqlstate '45000' set message_text = msg; 
    END IF; 
END $$ 
DELIMITER ;