How to Delete Column in SQL

In SQL, deleting a column from an existing table is a straightforward process, but it's important to understand the implications and the correct syntax involved. While there is no direct DELETE COLUMN command in SQL, we can achieve this by using the ALTER TABLE command combined with DROP COLUMN.

In this article, we'll explain how to delete a column in SQL, providing the syntax, practical examples, and best practices for safely removing unnecessary columns from your database.

Understanding Process of Deleting a Column in SQL

Deleting a column in SQL refers to the process of permanently removing a specific column from a table. This is done using the DROP COLUMN command, which is part of Data Definition Language (DDL) operations. Deleting columns that are no longer needed can significantly improve our database's performance, reduce storage consumption, and help maintain a clean and efficient schema.

Why Deleting a Column in SQL is Important

1. Data Optimization: Dropping columns that are no longer needed reduces the size of our table and optimizes storage space.
   
2. Improved Performance: Removing unnecessary columns can help improve the performance of SQL queries since the database engine doesn't have to retrieve and process unnecessary data.
   
3. Cleaner Database Schema: It helps in maintaining a clean and organized schema, especially when dealing with evolving data models.

Syntax

ALTER TABLE  Name_of_the_table
DROP COLUMN  Name_of_the_column ;

Key Terms

• ALTER TABLE: This clause is used to modify the structure of an existing table.
  
• DROP COLUMN: This specifies that a column should be deleted from the table.
  
• table_name: The name of the table from which the column will be removed.
  
• column_name: The name of the column that you want to delete.

Example 1: Deleting a Column from a Table

Let's walk through a practical example to demonstrate how to delete a column in SQL. In this scenario, we will create a table named GeeksforGeeks. This table consists of rank, name, age, monthly score, questions solved, and overall score as the columns of the table GeeksforGeeks.

Step 1: Create the Sample Table

CREATE TABLE GeeksforGeeks (
  rank int,
  name varchar(100),
  age int,
  monthly_score int,
  questions_solved int,
  overall_score int
);

Step 2: Insert Data into the Table

INSERT INTO GeeksforGeeks(rank,name,age,monthly_score,questions_solved,overall_score) 
VALUES (01, 'Vishu', 20 ,272 ,415 ,1448);
INSERT INTO GeeksforGeeks(rank,name,age,monthly_score,questions_solved,overall_score) 
VALUES (02, 'Kuntal', 20 ,271 ,410 ,1446);
INSERT INTO GeeksforGeeks(rank,name,age,monthly_score,questions_solved,overall_score) 
VALUES (03, 'Priyam', 20 ,270 ,408 ,1440);
INSERT INTO GeeksforGeeks(rank,name,age,monthly_score,questions_solved,overall_score) 
VALUES (04, 'Shailesh', 21 ,268 ,407 ,1438);
INSERT INTO GeeksforGeeks(rank,name,age,monthly_score,questions_solved,overall_score) 
VALUES (05, 'Avirup', 20 ,267 ,406 ,1437);
INSERT INTO GeeksforGeeks(rank,name,age,monthly_score,questions_solved,overall_score) 
VALUES (06, 'Neeraj', 21 ,265 ,405 ,1436);

Step 3: View the Data in the Table

SELECT * FROM GeeksforGeeks;

Output

Explanation:

In the "GeeksforGeeks" table, the "age" column holds no value to the table data. We can remove it using the DROP COLUMN command without affecting the rest of the table.

Step 4: Delete the age Column

If the age column is no longer needed, it can be dropped using the DROP COLUMN command.

ALTER TABLE GeeksforGeeks
DROP COLUMN age;

Step 5: Verify the Changes

Check the table structure and data after dropping the column.

SELECT * FROM GeeksforGeeks;

Output

Explanation:

The "age" column has been successfully removed from the table without affecting the other columns.

Example 2: Dropping Multiple Columns

In SQL, we can also drop multiple columns in a single query by separating column names with commas. This method simplifies queries by eliminating the need to write multiple ALTER TABLE statements, making the operation more efficient.

Step 1: Create a Table

CREATE TABLE Products (
  product_id INT,
  product_name VARCHAR(50),
  category VARCHAR(50),
  price DECIMAL(10, 2),
  stock_quantity INT
);

Step 2: Insert Sample Data

INSERT INTO Products (product_id, product_name, category, price, stock_quantity) 
VALUES 
  (1, 'Laptop', 'Electronics', 800.00, 50),
  (2, 'Desk', 'Furniture', 150.00, 20),
  (3, 'Chair', 'Furniture', 85.00, 100);

Step 3: Drop "category" and "stock_quantity" Columns

ALTER TABLE Products
DROP COLUMN category, stock_quantity;

Step 4: View the Updated Table

SELECT * FROM Products;

Output

Explanation:

Both the "category" and "stock_quantity" columns have been successfully removed, leaving only the relevant columns in the table.

Important Notes About Using DROP COLUMN in SQL

• Irreversible Change: Once we drop a column using the DROP COLUMN command, the data is permanently lost, and the column cannot be recovered.
  
• Foreign Key Constraints: If the column being dropped is referenced by foreign key constraints, we will need to drop those constraints before dropping the column.
  
• Large Tables: Dropping a column in a large table might take some time, depending on the size of the table and the database system.

Conclusion

The DROP COLUMN command is a useful tool for database administrators and developers to clean up unnecessary data and optimize table structures in SQL. By removing redundant or irrelevant columns, we can reduce storage space, improve query performance, and maintain a cleaner schema. However, it's essential to carefully review the column before dropping it since the operation is permanent.