Add Create A Composite Primary Key as a postgres til.

postgres/create-a-composite-primary-key.md

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+# Create A Composite Primary Key
+
+The unique identifier for a given row in a table is the *primary key*.
+Generally, a row can be uniquely identified by a single data point (such as
+an id), so the primary key is simply that single data point. In some cases,
+your data can be more appropriately uniquely identified by multiple values.
+This is where composite primary keys can lend a hand. Consider an example
+`plane_tickets` table where each ticket can be uniquely identified by the
+passenger and flight it is associated with:
+
+```sql
+create table plane_tickets (
+  passenger_id integer references passengers not null,
+  flight_id integer references flights not null,
+  confirmation_number varchar(6) not null,
+  seat_assignment varchar not null,
+  primary key (passenger_id, flight_id)
+);
+```