Question

I’m using PHPMyAdmin and I try to add the NOT NULL constraint to a column of my table.

PHPMyAdmin accepts my following query :

ALTER TABLE `wall` MODIFY `token_message` varchar(40) NOT NULL;

But I can still insert empty strings (=NULL), I don’t understand why.

PS : If you’re going to give me some other queries to add this constraint, note I’ve have tried these 3 which don’t work in my PHPMyAdmin (kind of error : #1064 – You have an error in your SQL syntax; check the manual) :

ALTER TABLE `wall` ALTER COLUMN `token_message` SET NOT NULL;
ALTER TABLE `wall` ALTER COLUMN `token_message` varchar(40) NOT NULL;
ALTER TABLE `wall` MODIFY `token_message` CONSTRAINTS token_message_not_null NOT NULL; 

Answer 1

You wrote, “I can still insert empty strings (=NULL),” which sounds like a misunderstanding. In SQL, an empty string does not evaluate to NULL, or vice versa. Try inserting an empty string and doing SELECT from wall where token_message is NULL. You should get zero rows back. Then try doing an insert where you specify NULL (unquoted) as the value for your column, and you should get the expected error message.

If those tests work as expected, then everything is fine, and your problem is actually that you want to prevent blank strings from being inserted. Check out this question for suggestions, or just check for blank strings during validation, before the query.