There are times when it doesn't make sense for a constraint to apply to all records in a table. For instance, if we have a table of pokemon, we may only want to apply a unique index constraint to the names of non-wild pokemon. This can be achieved with the following conditional constraint:
create unique index pokemons_names on pokemons (names)
where wild = false;If we try to insert a non-wild pokemon with a duplicate name, we will get an error. Likewise, if we try to update a pokemon with a duplicate name from wild to non-wild, we will get an error.