How to drop a column in SQL?

How to drop a column in SQL

Introduction

• To delete a column from an already created table, one needs to use the ALTER command along with the DROP COLUMN clause.

Syntax:

ALTER TABLE tablename DROP COLUMN ColumnName;

Where,

1. Tablename is the name of an already existing table from which you have to delete a column.
2. ColumnName is the name of the column which is to be deleted from an already existing table.

Example 1:

First, we will create a database with name “studentdb”. Then in that database we will create a table “student” and insert records into the table.

Now, we will delete a column ‘City’ from an existing table.

 mysql> USE studentdb;
 Database changed
 mysql> SELECT *FROM student;
 +---------+-----------+-----------+-------------+------+
 | Stud_ID | Stud_Name | Course_ID | Course_Name | City |
 +---------+-----------+-----------+-------------+------+
 |       1 | Prajakta  |       101 | DBMS        | NULL |
 |       2 | Shweta    |       102 | CN          | NULL |
 |       3 | Nikita    |       103 | OS          | NULL |
 |       4 | Ankita    |       104 | C           | NULL |
 |       5 | Ashmita   |       105 | JAVA        | NULL |
 +---------+-----------+-----------+-------------+------+
 5 rows in set (0.00 sec)
 mysql> ALTER TABLE student DROP COLUMN City;
 Query OK, 5 rows affected (0.29 sec)
 Records: 5  Duplicates: 0  Warnings: 0
 mysql> SELECT *FROM student;
 +---------+-----------+-----------+-------------+
 | Stud_ID | Stud_Name | Course_ID | Course_Name |
 +---------+-----------+-----------+-------------+
 |       1 | Prajakta  |       101 | DBMS        |
 |       2 | Shweta    |       102 | CN          |
 |       3 | Nikita    |       103 | OS          |
 |       4 | Ankita    |       104 | C           |
 |       5 | Ashmita   |       105 | JAVA        |
 +---------+-----------+-----------+-------------+
 5 rows in set (0.00 sec) 

Column ‘City’ is removed from an existing student table.

Example 2:

We will delete a column ‘Stud_ID’ along with its primary key.

 mysql> DESC student;
 +-------------+-------------+------+-----+---------+-------+
 | Field       | Type        | Null | Key | Default | Extra |
 +-------------+-------------+------+-----+---------+-------+
 | Stud_ID     | int(11)     | NO   | PRI | NULL    |       |
 | Stud_Name   | varchar(20) | YES  |     | NULL    |       |
 | Course_ID   | int(11)     | YES  |     | NULL    |       |
 | Course_Name | varchar(20) | YES  |     | NULL    |       |
 +-------------+-------------+------+-----+---------+-------+
 4 rows in set (0.02 sec)
 mysql> ALTER TABLE student DROP COLUMN Stud_ID;
 Query OK, 5 rows affected (0.28 sec)
 Records: 5  Duplicates: 0  Warnings: 0
 mysql> DESC student;
 +-------------+-------------+------+-----+---------+-------+
 | Field       | Type        | Null | Key | Default | Extra |
 +-------------+-------------+------+-----+---------+-------+
 | Stud_Name   | varchar(20) | YES  |     | NULL    |       |
 | Course_ID   | int(11)     | YES  |     | NULL    |       |
 | Course_Name | varchar(20) | YES  |     | NULL    |       |
 +-------------+-------------+------+-----+---------+-------+
 3 rows in set (0.01 sec) 

We have used DESC command in SQL to view the structure of student table. Before executing the ALTER command, there exists a primary key on ‘Stud_ID’. After executing the ALTER command along with the DROP column clause, ‘Stud_ID’ is removed from the table structure along with its primary key.