Add Adding Composite Uniqueness Constraints as a postgres til.

postgres/adding-composite-uniqueness-constraints.md

@@ -0,0 +1,22 @@
+# Adding Composite Uniqueness Constraints
+
+There are two ways in Postgres to create a composite uniqueness constraint,
+that is, a constraint that ensures that the combination of two or more
+values on a table only appear once. For the following two code snippets,
+assume that we have a table relating Pokemon and Trainers and that our
+domain restricts each Trainer to only having at most one of each Pokemon.
+
+The first approach is to create a `constraint` directly on the table:
+
+```sql
+alter table pokemons_trainers
+  add constraint pokemons_trainers_pokemon_id_trainer_id_key
+  unique (pokemon_id, trainer_id);
+```
+
+The second approach is to create a unique index:
+
+```sql
+create unique index pokemons_trainers_pokemon_id_trainer_id_idx
+  on pokemons_trainers (pokemon_id, trainer_id);
+```