Create a MySQL table named countries and enforce a constraint to prevent duplicate entries in the country_id column at runtime

MySQL Create Tables: Exercise-9 with Solution

9. Write a MySQL query to create a table named countries including columns 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:

-- Creating a table named 'countries' if it doesn't already exist to store information about countries

CREATE TABLE IF NOT EXISTS countries(
    -- Column to store the two-letter country code, marked as NOT NULL (required)
    COUNTRY_ID varchar(2) NOT NULL,

    -- Column to store the name of the country (up to 40 characters), marked as NOT NULL (required)
    COUNTRY_NAME varchar(40) NOT NULL,

    -- Column to store the region ID with decimal precision of 10, 0, marked as NOT NULL (required)
    REGION_ID decimal(10,0) NOT NULL,

    -- Adding a UNIQUE constraint on the COUNTRY_ID column to enforce uniqueness
    UNIQUE(COUNTRY_ID)
);

Let execute the above code in MySQL command prompt

Here is the structure of the table:

mysql> DESC countries;
+--------------+---------------+------+-----+---------+-------+
| Field        | Type          | Null | Key | Default | Extra |
+--------------+---------------+------+-----+---------+-------+
| COUNTRY_ID   | varchar(2)    | YES  |     | NULL    |       |
| COUNTRY_NAME | varchar(40)   | YES  |     | NULL    |       |
| REGION_ID    | decimal(10,0) | YES  |     | NULL    |       |
+--------------+---------------+------+-----+---------+-------+
3 rows in set (0.01 sec)

Explanation:

The provided MySQL code creates a table named "countries" if it doesn't already exist. The table has three columns:

• COUNTRY_ID: Stores two-letter country codes and is marked as NOT NULL (required).
• COUNTRY_NAME: Stores country names (up to 40 characters) and is marked as NOT NULL (required).
• REGION_ID: Stores region IDs with a decimal precision of 10, 0 and is marked as NOT NULL (required).

Additionally, a UNIQUE constraint is applied to the COUNTRY_ID column, ensuring that each country has a unique two-letter code within the table.

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