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please have a look at the following table:
name | x | y
---------+-----+------
foo | 3 | 5
bar | 45 | 99
foobar | 88 |
barfoo | 0 | 45
I want to add a constraint CHECK ( y > x ), but this obviously will fail due it is violated by the row ‘foobar’.
How do I create a constraint that says: check (y > x), but only if y not null?
In fact, you don’t really need to do anything else. A check constraint is satisfied if the check expression evaluates to true or the null value.
Your constraint CHECK ( y > x ) would work as is in your scenario because the row foobar would not violate the constraint since it evaluates to null