MySQL DROP COLUMN

 MySQL DROP COLUMN

Summary: in this tutorial, you will learn how to drop a column from a table using the MySQL DROP COLUMN statement.

Introduction to MySQL DROP COLUMN statement

In some situations, you want to remove one or more columns from a table. In such cases, you use the following ALTER TABLE DROP COLUMN statement:

ALTER TABLE table_name
DROP COLUMN column_name;

In this syntax:

• First, specify the name of the table that contains the column which you want to drop after the ALTER TABLE keywords.
• Second, specify the name of the column that you want to drop in theDROP COLUMN clause.

Note that the keyword COLUMN keyword in the DROP COLUMN a clause is optional so you can use the shorter statement as follows:

ALTER TABLE table_name
DROP column_name;

To remove multiple columns from a table using a single ALTER TABLE the statement, you use the following syntax:

ALTER TABLE table_name
DROP COLUMN column_name_1,
DROP COLUMN column_name_2,
...;

There are some important points you should remember before removing a column from a table:

• Removing a column from a table makes all database objects such as stored procedures, views, and triggers that referencing the dropped column invalid. For example, you may have a stored procedure that refers to a column. When you remove the column, the stored procedure becomes invalid. To fix it, you have to manually change the stored procedure’s code.
• The code from other applications that depends on the dropped column must be also changed, which takes time and effort.
• Dropping a column from a large table can impact the performance of the database during the removal time.

MySQL DROP COLUMN examples

First,  create a table named posts for the demonstration.

CREATE TABLE posts (
    id INT AUTO_INCREMENT PRIMARY KEY,
    title VARCHAR(255) NOT NULL,
    excerpt VARCHAR(400),
    content TEXT,
    created_at DATETIME,
    updated_at DATETIME
);

Next, use the ALTER TABLE DROP COLUMN statement to remove the excerpt column:

ALTER TABLE posts
DROP COLUMN excerpt;

Then, view the table structure using the DESCRIBE statement:

DESCRIBE posts;

After that, use the ALTER TABLE DROP COLUMN statement to drop the created_at and updated_at columns:

ALTER TABLE posts
DROP COLUMN created_at,
DROP COLUMN updated_at;

Finally, use the DESCRIBE statement to verify the removal:

DESCRIBE posts;

MySQL drop a column which is a foreign key example

If you remove the column that is a foreign key, MySQL will issue an error. Consider the following example.

First, create a table named categories:

CREATE TABLE categories (
    id INT AUTO_INCREMENT PRIMARY KEY,
    name VARCHAR(255)
);

Second, add a column named category_id to the posts table.

ALTER TABLE posts 
ADD COLUMN category_id INT NOT NULL;

Third, make the category_id column as a foreign key column that references to the id column of the categories table.

ALTER TABLE posts 
ADD CONSTRAINT fk_cat 
FOREIGN KEY (category_id) 
REFERENCES categories(id);

Fourth, drop the category_id column from the posts table.

ALTER TABLE posts
DROP COLUMN category_id;

MySQL issued an error message:

Error Code: 1553. Cannot drop index 'fk_cat': needed in a foreign key constraint

To avoid this error, you must remove the foreign key constraint before dropping the column.

In this tutorial, we have shown you how to use MySQL DROP COLUMN statement to remove one or more columns from a table.