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:

-- This SQL statement creates a new table named 'countries' if it does not already exist,
-- defining the structure and constraints for the table columns.

CREATE TABLE IF NOT EXISTS countries ( 
    COUNTRY_ID varchar(2) NOT NULL, -- Defines a column 'COUNTRY_ID' to store country IDs as strings with a maximum length of 2 characters. The column cannot contain NULL values.
    COUNTRY_NAME varchar(40) NOT NULL, -- Defines a column 'COUNTRY_NAME' to store country names as strings with a maximum length of 40 characters. The column cannot contain NULL values.
    REGION_ID decimal(10,0) NOT NULL, -- Defines a column 'REGION_ID' to store region IDs as decimal numbers with a precision of 10 digits and no decimal places. The column cannot contain NULL values.
UNIQUE(COUNTRY_ID) -- Defines a unique constraint on the 'COUNTRY_ID' column, ensuring that each value in this column is unique across all rows.
);

Explanation:

• The CREATE TABLE IF NOT EXISTS statement creates a new table only if it does not already exist in the database.
• The table is named 'countries'.
• Each column definition specifies the column name, data type, and optional constraints.
• varchar(2) indicates a variable-length character string with a maximum length of 2 characters for the 'COUNTRY_ID' column.
• varchar(40) indicates a variable-length character string with a maximum length of 40 characters for the 'COUNTRY_NAME' column.
• decimal(10,0) indicates a decimal number with a precision of 10 digits and no decimal places for the 'REGION_ID' column.
• The NOT NULL constraint ensures that none of the columns ('COUNTRY_ID', 'COUNTRY_NAME', and 'REGION_ID') can contain NULL values.
• The UNIQUE constraint ensures that each value in the 'COUNTRY_ID' column must be unique across all rows in the table.

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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