MySQL Default Record from Table

I use MySQL 5.6 and I have 3 tables. Two of them are a list of data, and the third table is used for a relationship between them. In the third table I would like to identify a default record for each id1, so I added a column named predt to let me know which record is the default one. Something like this:

id1 | id2 | predt
1   | 1   | 0
1   | 2   | 1
1   | 3   | 0

In this example I will know that the second row is the default one for id1 = 1.

Whats worries me is that it’s possible that more than one record could have the value 1 for column predt where id1 = 1.

I validate that this doesn’t happen, is the first thing I do in the SP that inserts or updates a record in this table. But if someone updates the table directly there is no restriction in the table to stop it from doing so (like a foreign key restriction).

create table table1 (
id int(10) not null auto_increment,
description varchar(100) not null,
primary key(id))
engine innodb,
default character set latin1;

create table table2 (
id int(10) not null auto_increment,
description varchar(100) not null,
primary key(id))
engine innodb,
default character set latin1;

create table table3 (
id1 int(10) not null,
id2 int(10) not null,
predt tinyint(1) not null default 0,
primary key(id1,id2))
engine innodb,
default character set latin1;

I always treated this case in the same way, and so far it has worked. But I would like to know if there is a better way to work with this kind of cases.

Answer

You should test if for that particular row already exists such predt = 1 In higher mysql Versions you could use a CHECK Constraint with the same purpose

DELIMITER $$
CREATE TRIGGER before_insert_table3 
BEFORE INSERT
ON table3 FOR EACH ROW
BEGIN
    IF NEW.predt = 1 then    
    IF EXISTS(SELECT 1 FROM table3 WHERE predt = 1 AND id1 = NEW.id1) THEN
        SIGNAL SQLSTATE '45000'
            SET MESSAGE_TEXT = ' predt 1 already exist';
    END IF; 
END IF;
END $$

DELIMITER ;

DELIMITER $$
CREATE TRIGGER before_update_table3 
BEFORE UPDATE
ON table3 FOR EACH ROW
BEGIN
    IF NEW.predt = 1 then    
        IF EXISTS(SELECT 1 FROM table3 WHERE predt = 1 AND id1 = NEW.id1) THEN
            SIGNAL SQLSTATE '45000'
                SET MESSAGE_TEXT = ' predt 1 already exist';
        END IF; 
    END IF;
END $$

DELIMITER ;