How do I add a custom CHECK constraint on a MySQL table?

I am having trouble with this table

CREATE TABLE `Participants` (
  `meetid` int(11) NOT NULL,
  `pid` varchar(15) NOT NULL,
  `status` char(1) DEFAULT NULL,
  PRIMARY KEY (`meetid`,`pid`),
  CONSTRAINT `participants_ibfk_1` FOREIGN KEY (`meetid`) REFERENCES `Meetings` (`meetid`) ON DELETE CASCADE
  CONSTRAINT `participants_ibfk_2` CHECK (status IN ('a','d','u'))
  CONSTRAINT `participants_ibfk_3` CHECK (pid IN (SELECT name FROM Rooms) OR pid IN (SELECT userid FROM People))
);

I want to have a foreign key constraint, and that works. Then, I also want to add a constraint to the attribute status so it can only take the values ‘a’, ‘d’ and ‘u’. It is not possible for me to set the field as Enum or set.

Can anyone tell me why this code does not work in MySQL?

Answer

Starting with version 8.0.16, MySQL has added support for CHECK constraints:

ALTER TABLE topic
ADD CONSTRAINT post_content_check
CHECK (
    CASE
        WHEN DTYPE = 'Post'
        THEN
            CASE
                WHEN content IS NOT NULL
                THEN 1
                ELSE 0
            END
        ELSE 1
    END = 1
);
 
ALTER TABLE topic
ADD CONSTRAINT announcement_validUntil_check
CHECK (
    CASE
        WHEN DTYPE = 'Announcement'
        THEN
            CASE
                WHEN validUntil IS NOT NULL
                THEN 1
                ELSE 0
            END
        ELSE 1
    END = 1
);

Previously, this was only available using BEFORE INSERT and BEFORE UPDATE triggers:

CREATE
TRIGGER post_content_check BEFORE INSERT
ON topic
FOR EACH ROW
BEGIN
   IF NEW.DTYPE = 'Post'
   THEN
       IF NEW.content IS NULL
       THEN
           signal sqlstate '45000'
           set message_text = 'Post content cannot be NULL';
       END IF;
   END IF;
END;
 
CREATE
TRIGGER post_content_update_check BEFORE UPDATE
ON topic
FOR EACH ROW
BEGIN
   IF NEW.DTYPE = 'Post'
   THEN
       IF NEW.content IS NULL
       THEN
           signal sqlstate '45000'
           set message_text = 'Post content cannot be NULL';
       END IF;
   END IF;
END;
 
CREATE
TRIGGER announcement_validUntil_check BEFORE INSERT
ON topic
FOR EACH ROW
BEGIN
   IF NEW.DTYPE = 'Announcement'
   THEN
       IF NEW.validUntil IS NULL
       THEN
           signal sqlstate '45000'
           set message_text = 'Announcement validUntil cannot be NULL';
       END IF;
   END IF;
END;
 
CREATE
TRIGGER announcement_validUntil_update_check BEFORE UPDATE
ON topic
FOR EACH ROW
BEGIN
   IF NEW.DTYPE = 'Announcement'
   THEN
       IF NEW.validUntil IS NULL
       THEN
           signal sqlstate '45000'
           set message_text = 'Announcement validUntil cannot be NULL';
       END IF;
   END IF;
END;