Add Create A Composite Primary Key as a postgres til.

README.md

@@ -105,6 +105,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 - [Checking The Type Of A Value](postgres/checking-the-type-of-a-value.md)
 - [Configure The Timezone](postgres/configure-the-timezone.md)
 - [Count Records By Type](postgres/count-records-by-type.md)
+- [Create A Composite Primary Key](postgres/create-a-composite-primary-key.md)
 - [Default Schema](postgres/default-schema.md)
 - [Defining Arrays](postgres/defining-arrays.md)
 - [Edit Existing Functions](postgres/edit-existing-functions.md)

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

@@ -0,0 +1,19 @@
+# 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)
+);
+```