Could I make a column in a table only allows one 'true' value and all other rows should be 'false'

Question

In MySQL, Can I put a restriction like below at database level?

I need to make sure 1. a specific column can only take either \'true\' or \'false\' values. 2. and e

Answer 1

In MySQL, a unique index will ignore any NULL values. So there's a bit of a hack you could use.

You could consider adding a nullable bit column (which can only have the value 1 or 0):

ALTER TABLE mytable 
  ADD COLUMN `is_default` BIT NULL,
  ADD UNIQUE INDEX `is_default_UNIQUE` (`is_default` ASC);

At this point, one row can be 1 (TRUE), and attempting to add another row that is also TRUE, will result in an error:

1062: Duplicate entry '\x01' for key 'is_default_UNIQUE'

The only catch is that all other rows need to be NULL and cannot be false as that would also count as a unique value.

(This really isn't what indexes are meant to be used for though)