MySQL错误1241操作数应在IF EXISTS上包含1列

Question

I have been getting this Error 1241 Operand should contain 1 column(s) in a Stored Procedure in MySQL.

This error has been highly cited in other posts, but none of these fitted my situation, and so I have created a question & answer to help others who may be struggling with the source of this error, and in which the other answers do not lead to the solution.

Here is a reduction of my code:

DELIMITER //
DROP PROCEDURE IF EXISTS TEST
//
CREATE PROCEDURE TEST()
BEGIN
    IF NOT (SELECT * FROM categories) THEN
        SELECT 1;
    END IF;
END;
//

CALL TEST();

This compiles OK, but generates the runtime error 1241 error.

Answer 1

The error is caused by my failing to include the EXISTS keyword, and this statement should have read:

IF NOT EXISTS (SELECT * FROM categories) THEN

The challenge is that the original compiles without an error.

Whereas this clearly falls under the heading of "silly mistakes", it is often these types of mistakes which hold us up in our work, when we look in the wrong place for a solution, and it is my experience that these kinds of silly mistakes are made by both novice and experienced developers.

I hope that this may help to reduce your time wasted in searching for more complex solutions when this simple fix will suffice.