A constraint to prevent the insert of an empty string in MySQL

Question

In this question I learned how to prevent the insert of a NULL value. But, unfortunately, an empty string is being inserted anyway. Apart from preventing this on the PHP sid

Answer 1

You could use triggers to prevent insertion of blankstring.

It's not fast, not very concise and not pretty, but...

Example:

1. Create your table:

   mysql> create table yar (val VARCHAR(25) not null);
   Query OK, 0 rows affected (0.02 sec)
   

2. Create your 'before insert' trigger to check for blankstring and disallow.

   mysql> delimiter $
   
   mysql> create trigger foo before insert on yar
       -> for each row
       -> begin
       -> if new.val = '' then
       -> signal sqlstate '45000';
       -> end if;
       -> end;$
   Query OK, 0 rows affected (0.01 sec)
   
   mysql> delimiter ;
   

3. Try to insert null and blankstring into your column:

   mysql> insert into yar values("");
   ERROR 1644 (45000): Unhandled user-defined exception condition
   
   mysql> insert into yar values(NULL);
   ERROR 1048 (23000): Column 'val' cannot be null
   
   mysql> insert into yar values ("abc");
   Query OK, 1 row affected (0.01 sec)
   
   mysql> select * from yar;
   +-----+
   | val |
   +-----+
   | abc |
   +-----+
   1 row in set (0.00 sec)
   

Answer 2

Normally you would do that with CHECK constraint:

foo_test VARCHAR(50) NOT NULL CHECK (foo_test <> '')

Prior to Version 8.0 MySQL had limited support for constraints. From MySQL Reference Manual:

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

If you must stick to an old version use triggers as a workaround, as people have pointed out.

In future, you may want to take a look at PostgreSQL, which is considered to have better support for data integrity (among other things) by many people.