First I created a table like
CREATE TABLE Customer (
SD integer CHECK (SD > 0),
Last_Name varchar (30),
First_Name varchar(30)
);
and then inserted values in that table
INSERT INTO Customer values ('-2','abc','zz');
MySQL doesn't show an error, it accepted the values.
MySQL 8.0.16 is the first version that supports CHECK constraints.
Read https://dev.mysql.com/doc/refman/8.0/en/create-table-check-constraints.html
If you use MySQL 8.0.15 or earlier, the MySQL Reference Manual says:
The
CHECK
clause is parsed but ignored by all storage engines.
Try a trigger...
mysql> delimiter //
mysql> CREATE TRIGGER trig_sd_check BEFORE INSERT ON Customer
-> FOR EACH ROW
-> BEGIN
-> IF NEW.SD<0 THEN
-> SET NEW.SD=0;
-> END IF;
-> END
-> //
mysql> delimiter ;
Hope that helps.