Question

I am maintaing a project. There is a table called ‘timeblock’, it has a field called ‘break_minutes’, in the legacy design, the type for ‘break_minutes’ is int, and default value is NULL.

This design is not good for my new task, because i will use ‘break_minutes’ to do some math functions. like sum. So i have already changed the database schema and the default value to 0 not Null.

So I have to change all the data whose ‘break_minutes’ value is NULL to 0.

So I write the sql:

update timeblock set break_minutes=0 where break_minutes=NULL

and the result is empty.

if I write sql like:

update timeblock set break_minutes=0 where break_minutes=NULL

the result is only for the rows whose value is 0.

Both two sqls can’t achieve my expectations.

Anyone who has the idea for that? thanks.

Answer 1

To check whether a value is NULL, use the special comparison predicates IS NULL instead of using comparison operators.
Comparison operators return unknown when comparing anything with NULL.

UPDATE timeblock SET break_minutes=0 WHERE break_minutes IS NULL