Creating Conditional Constraints in PostgreSQL

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 in PostgreSQL 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.

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