Add Creating Conditional Constraints as a postgres til.

postgres/creating-conditional-constraints.md

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+# Creating Conditional Constraints
+
+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:
+
+```sql
+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.
+
+[source](http://www.postgresguide.com/performance/conditional.html)