MySQL Alter Table Statement Exercises: Add a foreign key on job_id column of job_history table referencing to the primary key job_id of jobs table

MySQL Alter Table Statement: Exercise-11 with Solution

Write a MySQL statement to add a foreign key on job_id column of job_history table referencing to the primary key job_id of jobs table.

Here is the structure of the table jobs and job_history.

mysql> SHOW COLUMNS FORM jobs;
+------------+--------------+------+-----+---------+-------+
| Field      | Type         | Null | Key | Default | Extra |
+------------+--------------+------+-----+---------+-------+
| JOB_ID     | int(11)      | NO   | PRI | NULL    |       |
| JOB_TITLE  | varchar(35)  | NO   |     | NULL    |       |
| MIN_SALARY | decimal(6,0) | YES  |     | NULL    |       |
| MAX_SALARY | decimal(6,0) | YES  |     | NULL    |       |
+------------+--------------+------+-----+---------+-------+

mysql> SHOW COLUMNS FROM job_history;
+---------------+---------+------+-----+---------+-------+
| Field         | Type    | Null | Key | Default | Extra |
+---------------+---------+------+-----+---------+-------+
| EMPLOYEE_ID   | int(11) | NO   |     | NULL    |       |
| START_DATE    | date    | YES  |     | NULL    |       |
| HIRE_DATE     | date    | YES  |     | NULL    |       |
| JOB_ID        | int(11) | NO   |     | NULL    |       |
| DEPARTMENT_ID | int(11) | NO   |     | NULL    |       |
+---------------+---------+------+-----+---------+-------+

Code:

-- This SQL statement is used to alter the 'job_history' table by adding a foreign key constraint.
-- The foreign key is added on the 'job_id' column, referencing the 'job_id' column in the 'jobs' table.
	
ALTER TABLE job_history

-- Add a foreign key constraint to the 'job_id' column in the 'job_history' table.
ADD FOREIGN KEY(job_id)

-- Specify the referenced table and column for the foreign key constraint.
REFERENCES jobs(job_id);

Let execute the above code in MySQL command prompt

Now see the structure of the table job_history after being altered.

mysql> SHOW COLUMNS FROM job_history;
+---------------+---------+------+-----+---------+-------+
| Field         | Type    | Null | Key | Default | Extra |
+---------------+---------+------+-----+---------+-------+
| EMPLOYEE_ID   | int(11) | NO   |     | NULL    |       |
| START_DATE    | date    | YES  |     | NULL    |       |
| HIRE_DATE     | date    | YES  |     | NULL    |       |
| JOB_ID        | int(11) | NO   | MUL | NULL    |       |
| DEPARTMENT_ID | int(11) | NO   |     | NULL    |       |
+---------------+---------+------+-----+---------+-------+

Now see the created index file.

mysql> SHOW INDEX FROM job_history;
+-------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table       | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| job_history |          1 | JOB_ID   |            1 | JOB_ID      | A         |        NULL |     NULL | NULL   |      | BTREE      |         |
+-------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+

Explanation:

Here's a brief explanation of the above MySQL code:

• ALTER TABLE job_history: This part of the statement indicates that you want to make changes to the structure of the 'job_history' table.
• ADD FOREIGN KEY(job_id): It adds a foreign key constraint to the 'job_id' column in the 'job_history' table.
• REFERENCES jobs(job_id);: This specifies the referenced table and column for the foreign key constraint. It indicates that the 'job_id' column in 'job_history' should refer to the 'job_id' column in the 'jobs' table. This ensures that values in the 'job_id' column of 'job_history' must exist in the 'job_id' column of the 'jobs' table. This establishes a relationship between the two tables.

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Next: Write a SQL statement to add a foreign key constraint named fk_job_id on job_id column of job_history table referencing to the primary key job_id of jobs table.