Conditional Not Null Constraint

Question

Is there a way to model the following behaviour with constraint checks (or other functions) in postgres SQL:

I have two columns:

• oncost (boolean)
• oncostdescription (varchar)

If oncost is true, i want oncostdescription to be mandatory (NOT NULL, NOT EMPTY)

If concost is false, i want oncostdescription not to be mandatory (NULL, EMPTY)

EDIT:
I forgot to state my postgres version being lower than 12.
Some folks here came up with a cool feature called generated columns.
Seems pretty cool. But only if you have 12+

Answer 1

You can use a check constraint:

create table mytable (
    oncost boolean
    oncostdescription varchar(50),
    constraint ck_mytable check(
        not oncost 
        or not (oncostdescription is null or oncostdescription = '')
    )
)

This phrases as: either the boolean flag is false, or the description is neither null nor empty.

You could also express this as:

create table mytable (
    oncost boolean
    oncostdescription varchar(50),
    constraint ck_mytable check(
        not (
            oncost 
            and (oncostdescription is null or oncostdescription = '')
        )
    )
)