MySql workbench CHECK constraint [duplicate]

问题:

This question already has answers here:

CHECK constraint in MySQL is not working (8 answers)

Closed last year.

Here I want to create 2 CHECK constraint before the record insert to the database.

ALTER TABLE SubjectEnrollment
ADD CONSTRAINT register CHECK (register <= classSize AND register >=0),
ADD CONSTRAINT available CHECK (available <= classSize AND available >= 0);

1. register attribute should not more than classSize attribute and less than 0.
2. available attribute should not more than classSize attribte and less than 0.

When I type in this syntax in MySql Workbench, it complaints "Syntax Error: unexpected 'CHECK' (check)'. How should I add these, using TRIGGER?

Thank you.

回答1:

Since MySQL does not support check, you need a trigger for that. Something like this CREATE trigger:

delimiter $$
CREATE TRIGGER some_trigger_name
BEFORE INSERT ON SubjectEnrollment
FOR EACH ROW
BEGIN    
    IF (NEW.register > NEW.classSize OR NEW.register < 0)        
        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'invalid data';
    END IF;
END
$$

You need to define the same trigger for UPDATEs.

回答2:

A bit long for a comment.

MySQL supports the syntax for check, but in the create table:

create table (
    . . . 
    CHECK (register <= classSize AND register >= 0)
);

Check is not actually implemented in any engines, so this does nothing. Presumably for that reason, it is not part of the constraint syntax and hence not available for add constraint.