MySQL触发器约束检查

Question

I am not an expert with databases and I need help while I am doing my database project. I am using phpmyadmin version 4.1.14 as my database server. I am required to have a constraint which will make sure that a reader cannot borrow more than 10 books from the library. I am trying to set up a trigger that will run before the BORROWED table is updated to make sure the reader did not borrowed more than 10 books. If the reader has borrowed 10 books already I want to display a message.

I can't get this to work as I keep getting error. Here is my SQL for trigger

BEGIN
DECLARE msg varchar(255);
SELECT Count(ReaderID) as Counted from BORROWED 
WHERE RDateTime=null
IF Counted=10 THEN
SET msg='Reader cannot borrow more than 10 books';
SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT=msg;
END IF;
END

This is the error message I get. MySQL said: #1064 You have an error in your SQL syntax; check the manual that corresponds to you MySQL server version for the right syntax yo use near 'IF Counted=10 THEN SET msq='REader cannot borrow more than 10 books'; SIGNAL S' at line 5

Thank you for help !

Answer 1

Try defining Counted as a variable:

BEGIN
    DECLARE msg varchar(255);
    IF 10 <= (select count(*) from borrowed where rDateTime is NULL) THEN
        SET msg = 'Reader cannot borrow more than 10 books';
        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT=msg;
    END IF;
END;

Note that I also changed the = NULL to is NULL , the correct way to do a NULL comparison.