MySQL Alter Table Statement Exercises: Drop the index indx_job_id from job_history table

MySQL Alter Table Statement: Exercise-15 with Solution

Write a MySQL statement to drop the index indx_job_id from job_history table.

Here is the structure of the job_history and index file of the table job_history.

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


mysql> SHOW INDEXES FROM job_history;
+-------------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table       | Non_unique | Key_name    | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| job_history |          0 | PRIMARY     |            1 | EMPLOYEE_ID | A         |           0 |     NULL | NULL   |      | BTREE      |         |
| job_history |          1 | indx_job_id |            1 | JOB_ID      | A         |           0 |     NULL | NULL   |      | BTREE      |         |
+-------------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+

Code:

 -- This SQL statement is used to alter the 'job_history' table by dropping (removing) an existing index.
-- The index named 'indx_job_id' is being removed.

ALTER TABLE job_history

-- Drop the index named 'indx_job_id' from the 'job_history' table.
DROP INDEX indx_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   | PRI | NULL    |       |
| START_DATE    | date    | YES  |     | NULL    |       |
| HIRE_DATE     | date    | YES  |     | NULL    |       |
| JOB_ID        | int(11) | NO   |     | NULL    |       |
| DEPARTMENT_ID | int(11) | NO   |     | NULL    |       |
+---------------+---------+------+-----+---------+-------+

Now see the index file.

mysql> SHOW INDEXES FROM job_history;
+-------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table       | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| job_history |          0 | PRIMARY  |            1 | EMPLOYEE_ID | A         |           0 |     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.
• DROP INDEX indx_job_id;: It drops (removes) the index named 'indx_job_id' from the 'job_history' table. The 'indx_job_id' is the name of the index created earlier.

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