PostgreSQL Alter Table: Alter a table to add a primary key for a combination of columns

7. Write a SQL statement to add a primary key for a combination of columns location_id and country_id.

Here is the structure of the table locations.

postgres=# \d locations

     Column     |         Type          | Modifiers
----------------+-----------------------+-----------
 location_id    | numeric(4,0)          |
 street_address | character varying(40) |
 postal_code    | character varying(12) |
 city           | character varying(30) |
 state_province | character varying(25) |
 country_id     | character varying(2)  |

Now execute the following statement.

Sample Solution:

Code:

ALTER TABLE locations
ADD PRIMARY KEY(location_id,country_id);

Output:

Now see the structure of the table locations after alteration.

postgres=# \d locations

     Column     |         Type          | Modifiers
----------------+-----------------------+-----------
 location_id    | numeric(4,0)          | not null
 street_address | character varying(40) |
 postal_code    | character varying(12) |
 city           | character varying(30) |
 state_province | character varying(25) |
 country_id     | character varying(2)  | not null
Indexes:
    "locations_pkey" PRIMARY KEY, btree (location_id, country_id)

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