I have a table with columns a b and c, and if c is false then I only want to allow insertions if columns a and b are unique, but if c is true then a and b do not need to be unique.
Example:
There can only be one (foo, bar, false) in the table, but no limit on how many (foo, bar, true) there can be.
I tried something like CONSTRAINT blah UNIQUE (a,b) AND CHECK (C is TRUE) but I can't figure out the correct syntax.