Constraint or trigger greater than 18 SQL

There are at least 3 problems with your current solution:

1. CHECK constraint does not do anything in most current versions

In all versions of MySQL, as well as all versions of MariaDB prior to 10.2.1, the CHECK constraints do nothing. The syntax exists, but, literally, it does not do anything.

Reference :

  

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

translated (more or less):

  

The clause CHECK is analyzed, but ignored by all storage engines.

2. CHECK constraint only accepts deterministic functions

Starting with MariaDB 10.2.1, you can finally use the CHECK constraints like the other databases But still, there are 2 problems:

3. The logic is not correct

As Pablo Claus commented, even if the CHECK constraint accepts and uses your expression correctly, the logic you have is not correct, because it does not take into account the month and the day, but only the year.

To obtain the age of a person from a date of birth, the correct way to do it is this way:

timestampdiff(year, fechanac, now())

Solution

For the reasons mentioned above, in your case, it seems to me that the only correct solution is to use triggers to carry out the validation. Here is a model of how to do it:

delimiter $$
create procedure sp_check_cns_mayor18(in fechanac date)
begin
  if timestampdiff(year, fechanac, now()) < 18 then
    signal sqlstate '45000' set message_text = 'Votante debe tener 18 años o más.';
  end if;
end
$$

create trigger instrg_check_cns_mayor18
before insert on votantes
for each row
begin
  call sp_check_cns_mayor18(new.fechanac);
end
$$

create trigger updtrg_check_cns_mayor18
before update on votantes
for each row
begin
  call sp_check_cns_mayor18(new.fechanac);
end
$$
delimiter ;

Test with CURDATE() instead of NOW() :

ALTER TABLE votantes
ADD CONSTRAINT cns_mayor18 CHECK ((YEAR(CURDATE()) - YEAR(fechanac)) >= 18);