Write a postgre SQL statement to insert a record into the table countries to ensure that, at country_id and the region_id combination will be entered once in the table

Here is the code to create a sample table countries:

CREATE TABLE countries ( 
COUNTRY_ID integer NOT NULL,
COUNTRY_NAME varchar(40) NOT NULL,
REGION_ID integer NOT NULL,
PRIMARY KEY (COUNTRY_ID,REGION_ID)
);

Now insert the value into the table countries :

INSERT INTO countries VALUES(501,'India',185);

Here is the command to see the list of inserting rows :

postgres=# SELECT * FROM countries;
 country_id | country_name | region_id
------------+--------------+-----------
        501 | India        |       185
(1 row)

Now try to insert another row with the same combination of values for country_id and region_id, and see what happened:

postgres=# INSERT INTO kan VALUES(501,'Italy',185);
ERROR:  duplicate key value violates unique constraint "countries_pkey"
DETAIL:  Key (country_id, region_id)=(501, 185) already exists.