PostgreSQL Create Table: Create a table to restrict the insertion of duplicate value for a specified column

8. Write a SQL statement to create a table named countries, including country_id, country_name and region_id and make sure that no duplicate data against column country_id will be allowed at the time of insertion.

Sample Solution:

Code:

CREATE TABLE IF NOT EXISTS countries ( 
COUNTRY_ID varchar(2) NOT NULL,
COUNTRY_NAME varchar(40) NOT NULL,
REGION_ID decimal(10,0) NOT NULL,
UNIQUE(COUNTRY_ID)
);

This example can also be written like below.

Code:

CREATE TABLE IF NOT EXISTS countries ( 
COUNTRY_ID varchar(2) NOT NULL UNIQUE,
COUNTRY_NAME varchar(40) NOT NULL,
REGION_ID decimal(10,0) NOT NULL
);

Output:

postgres=# CREATE TABLE IF NOT EXISTS countries (
postgres(# COUNTRY_ID varchar(2) NOT NULL,
postgres(# COUNTRY_NAME varchar(40) NOT NULL,
postgres(# REGION_ID decimal(10,0) NOT NULL,
postgres(# UNIQUE(COUNTRY_ID)
postgres(# );
CREATE TABLE

Here is the command to see the structure of the table :

postgres=# \d countries
           Table "public.countries"
    Column    |         Type          | Modifiers
--------------+-----------------------+-----------
 country_id   | character varying(2)  | not null
 country_name | character varying(40) | not null
 region_id    | numeric(10,0)         | not null
Indexes:
    "countries_country_id_key" UNIQUE CONSTRAINT, btree (country_id)

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Previous: Write a SQL statement to create a table named countries, including country_id, country_name and region_id and make sure that no countries except Italy, India and China will be entered in the table.
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