How to create a “unique” constraint on a boolean MySQL column?

Question

I would like to add a BOOLEAN column to a MySQL table which will be named is_default. In this column, only one record can have is_default

Answer 1

Check constraints are not supported in MySQL, this is the solution using trigger:

    create table if not exists myTable (
        id int not null auto_increment primary key,
        is_default bit not null
    ) engine=innodb;

    select 'create trigger tbi_myTable';
    drop trigger if exists tbi_myTable;
    delimiter //
    create trigger tbi_myTable 
        before insert on myTable 
        for each row
    begin
        if (select count(1) from myTable where is_default=true) > 0 && NEW.is_default then
        -- Signal is only in 5.6 and above use another way to raise an error: if less than 5.6
            SIGNAL SQLSTATE '50000' SET MESSAGE_TEXT = 'Cannot insert into myTable only one row with is_default true is allowed!';
        end if;
    END //
    delimiter ;

    insert into myTable (is_default) values (false);
    insert into myTable (is_default) values (true);
    insert into myTable (is_default) values (false);
    insert into myTable (is_default) values (false);
    -- This will generate an error
    insert into myTable (is_default) values (true);
    insert into myTable (is_default) values (false);


    select * from myTable;
    -- will give
    /*
    id  is_default
    1   false
    2   true
    3   false
    4   false
    */