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:

-- 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 UNIQUE PRIMARY KEY, -- Defines a column 'COUNTRY_ID' to store country IDs as strings with a maximum length of 2 characters. The column cannot contain NULL values, and each value must be unique across all rows. Additionally, it is set as the primary key for the table.
    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.
);

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 'COUNTRY_ID', 'COUNTRY_NAME', and 'REGION_ID' columns cannot contain NULL values.
• The UNIQUE constraint ensures that each value in the 'COUNTRY_ID' column must be unique across all rows in the table.
• The PRIMARY KEY constraint sets the 'COUNTRY_ID' column as the primary key for the table, ensuring uniqueness and providing a unique identifier for each row in the table.

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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Previous: Write a SQL statement to create a table named jobs, including job_id, job_title, min_salary and max_salary, and make sure that, the default value for job_title is blank and min_salary is 8000 and max_salary is NULL will be entered automatically at the time of insertion if no value assigned for the specified columns.
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.