Question

I have a table with which I want to add a check constraint after it has already been created. I did some googling and found some answers, but they don’t appear to work. Here is what I have tried:

ALTER TABLE noise_table 
    ADD CONSTRAINT chk_level CHECK (noise_level >= 0 and noise_level <= 0.1);

However, after running this statement, I can still update and insert outside of the checked interval, so it doesn’t appear to be working at all. What am I missing here?

Answer 1

As documented under ALTER TABLE Syntax:

The CHECK clause is parsed but ignored by all storage engines.

You could instead define BEFORE INSERT and BEFORE UPDATE triggers that raise errors in the event that your criteria are not met:

DELIMITER ;;

CREATE TRIGGER chk_insert_level BEFORE INSERT ON noise_table FOR EACH ROW
IF NOT (NEW.noise_level >= 0 AND NEW.noise_level <= 0.1) THEN
  CALL invalid_noise_level;  -- non-existent procedure, raises an error
END IF;;

CREATE TRIGGER chk_update_level BEFORE UPDATE ON noise_table FOR EACH ROW
IF NOT (NEW.noise_level >= 0 AND NEW.noise_level <= 0.1) THEN
  CALL invalid_noise_level;  -- non-existent procedure, raises an error
END IF;;

DELIMITER ;