Where I work, in our database, we use 3 to indicate true and 7 to indicate false and 0 is true but null is false

In another table, we use 'P' to indicate true and 'I' to indicate false and 'Y' is also false and null is false

And the most used table, we also use 'Y' to indicate yes and 'N' to indicate no, but null is also Yes.

We also store integers as varchar in a live table, but stays an integer in all the other tables. I hope I'm not there when the number of digits exceeds the varchar limit.

These are all live and used in production all created by my boss, the head of IT.