PostgreSQL Create Table: Create a table to restrict one of the columns to contain any duplicate values

10. Write a SQL statement to create a table named countries, including columns country_id, country_name and region_id and make sure that the country_id column will be a key field which will not contain any duplicate data at the time of insertion.

Sample Solution:

Code:

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

The above statement can be written like below.

Code:

CREATE TABLE countries (
COUNTRY_ID varchar(2) NOT NULL DEFAULT '',
COUNTRY_NAME varchar(40) DEFAULT NULL,
REGION_ID decimal(10,0) DEFAULT NULL,
PRIMARY KEY (COUNTRY_ID));

Output:

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

Here is the command to see the structure of the created 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_pkey" PRIMARY KEY, btree (country_id)

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Next: Write a SQL statement to create a table countries including columns country_id, country_name and region_id and make sure that the column country_id will be unique and store an auto-incremented value.