Add Creating Conditional Constraints as a postgres til.

README.md

@@ -128,6 +128,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 - [Count Records By Type](postgres/count-records-by-type.md)
 - [Create A Composite Primary Key](postgres/create-a-composite-primary-key.md)
 - [Create hstore From Two Arrays](postgres/create-hstore-from-two-arrays.md)
+- [Creating Conditional Constraints](postgres/creating-conditional-constraints.md)
 - [Default Schema](postgres/default-schema.md)
 - [Defining Arrays](postgres/defining-arrays.md)
 - [Edit Existing Functions](postgres/edit-existing-functions.md)

postgres/creating-conditional-constraints.md

@@ -0,0 +1,17 @@
+# 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)