DELIMITER //

CREATE TRIGGER borrow_insert_2
AFTER INSERT ON borrow
FOR EACH ROW
BEGIN
DECLARE @num INT ;
SET @num=(SELECT num FROM book WHERE bookId =NEW.bookId);
IF @num<=0 THEN
DELETE FROM borrow WHERE bookId=NEW.bookId ;
ELSE
UPDATE book SET num=num-1 WHERE bookId=NEW.bookId ;
END IF ;
END //

DELIMITER ;
还是报错了
1 queries executed, 0 success, 1 errors, 0 warnings

查询：SET @num=(SELECT num FROM book WHERE bookId =NEW.bookId); IF @num<=0 THEN delete from borrow where bookId=NEW.bookId ; ELSE UPDA...

错误代码： 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 'IF @num<=0 THEN
delete from borrow where bookId=NEW.bookId ;
ELSE
UPDATE ' at line 2

执行耗时 : 0 sec
传送时间 : 0 sec
总耗时 : 0 sec
这个是borrow表
CREATE TABLE borrow(
studentId CHAR(7) NOT NULL,
bookId INT PRIMARY KEY,
borrowDate DATE,
state VARCHAR(3)CHECK (state IN('已还','未还')),
FOREIGN KEY (studentId) REFERENCES students(studentId),
FOREIGN KEY (bookId)REFERENCES book(bookId)
)

@zqhwmy:

声明的时候，变量前面别加@

还有你这里用bookId定位删除不对吧，把之前的记录都删了

DELIMITER //

CREATE TRIGGER borrow_insert_2
AFTER INSERT ON borrow
FOR EACH ROW
BEGIN
DECLARE num INT ;
SET @num=(SELECT num FROM book WHERE bookId =NEW.bookId);
IF @num<=0 THEN
DELETE FROM borrow WHERE bookId=NEW.bookId ;
ELSE
UPDATE book SET num=num-1 WHERE bookId=NEW.bookId ;
END IF ;
END //

DELIMITER ;

@foreversag: 可以了，非常感谢