Create a MySQL table named 'countries' with a unique combination of columns 'country_id' and 'region_id'

MySQL Create Tables: Exercise-13 with Solution

13. Write a MySQL query to create a table countries including columns country_id, country_name and region_id and make sure that the combination of columns country_id and region_id will be unique.

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), UNIQUE, with a default value of an empty string
    COUNTRY_ID varchar(2) NOT NULL UNIQUE DEFAULT '',

    -- Column to store the name of the country (up to 40 characters), with a default value of NULL
    COUNTRY_NAME varchar(40) DEFAULT 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,

    -- Defining a composite PRIMARY KEY on COUNTRY_ID and REGION_ID columns
    PRIMARY KEY (COUNTRY_ID,REGION_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)    | NO   | PRI |         |       |
| COUNTRY_NAME | varchar(40)   | YES  |     | NULL    |       |
| REGION_ID    | decimal(10,0) | YES  |     | NULL    |       |
+--------------+---------------+------+-----+---------+-------+
3 rows in set (0.01 sec)

Explanation:

The said 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, marked as NOT NULL, UNIQUE, and has a default value of an empty string.
• COUNTRY_NAME: Stores the names of countries (up to 40 characters) with a default value of NULL.
• REGION_ID: Stores region IDs with decimal precision of 10, 0, marked as NOT NULL.

Additionally, a composite PRIMARY KEY is defined on the 'COUNTRY_ID' and 'REGION_ID' columns.

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Previous: 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.
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