MySQL Alter Table Statement Exercises: 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
MySQL Alter Table Statement: Exercise-12 with Solution
Write a MySQL 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.
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 named foreign key constraint.
-- The foreign key is added on the 'job_id' column, referencing the 'job_id' column in the 'jobs' table.
-- Additional options are specified for the foreign key constraint, including ON UPDATE RESTRICT and ON DELETE CASCADE.
ALTER TABLE job_history
-- Add a named foreign key constraint 'fk_job_id' to the 'job_id' column in the 'job_history' table.
ADD CONSTRAINT fk_job_id
-- Specify that the foreign key is on the 'job_id' column.
FOREIGN KEY (job_id)
-- Specify the referenced table and column for the foreign key constraint.
REFERENCES jobs(job_id)
-- Specify the action to be taken when the referenced column is updated (RESTRICT).
ON UPDATE RESTRICT
-- Specify the action to be taken when the referenced row is deleted (CASCADE).
ON DELETE CASCADE;
Let execute the above code in MySQL command prompt
Now see the structure of the table locations 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 | fk_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 CONSTRAINT fk_job_id: This specifies the action to be taken. It adds a named foreign key constraint 'fk_job_id' to the 'job_id' column in the 'job_history' table.
- FOREIGN KEY (job_id): This part of the statement specifies that the foreign key is on the 'job_id' column.
- REFERENCES jobs(job_id): This part of the statement 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.
- ON UPDATE RESTRICT: In this case, the update is restricted, meaning that the update is not allowed if it breaks the foreign key relationship.
- ON DELETE CASCADE: In this case, a cascade delete is performed, meaning that if a row in 'jobs' is deleted, the corresponding rows in 'job_history' with matching 'job_id' values will also be deleted.
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Previous: Write a SQL statement to add a foreign key on job_id column of job_history table referencing to the primary key job_id of jobs table.
Next: Write a SQL statement to drop the existing foreign key fk_job_id from job_history table on job_id column which is referencing to the job_id of jobs table.
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https://w3resource.com/mysql-exercises/alter-table-statement/alter-table-exercise-12.php
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