Soft delete pattern concept in SQL

Introduction

Soft delete lets you hide data instead of removing it. This helps keep records safe and recoverable.

When you want to keep user accounts but mark them as inactive.

When deleting orders but still need to keep history for reports.

When you want to undo deletions if done by mistake.

When you need to keep audit trails for compliance.

When you want to avoid breaking links to deleted data.

Syntax

SQL

ALTER TABLE table_name ADD COLUMN is_deleted BOOLEAN DEFAULT FALSE;

-- To soft delete a row:
UPDATE table_name SET is_deleted = TRUE WHERE id = some_id;

-- To select only active rows:
SELECT * FROM table_name WHERE is_deleted = FALSE;

The is_deleted column marks if a row is deleted without removing it.

Queries should filter out rows where is_deleted is TRUE to hide deleted data.

Examples

Add a column to mark users as deleted or active.

SQL

ALTER TABLE users ADD COLUMN is_deleted BOOLEAN DEFAULT FALSE;

Soft delete the user with ID 5 by marking them as deleted.

SQL

UPDATE users SET is_deleted = TRUE WHERE user_id = 5;

Select only users who are not deleted.

SQL

SELECT * FROM users WHERE is_deleted = FALSE;

Sample Program

This example creates a products table with a soft delete column. It inserts three products, soft deletes one, and then selects only the active products.

SQL

CREATE TABLE products (
  product_id INT PRIMARY KEY,
  name VARCHAR(50),
  price DECIMAL(10,2),
  is_deleted BOOLEAN DEFAULT FALSE
);

INSERT INTO products (product_id, name, price) VALUES
(1, 'Pen', 1.20),
(2, 'Notebook', 2.50),
(3, 'Eraser', 0.80);

-- Soft delete product with product_id = 2
UPDATE products SET is_deleted = TRUE WHERE product_id = 2;

-- Select only active products
SELECT product_id, name, price FROM products WHERE is_deleted = FALSE ORDER BY product_id;

Important Notes

Soft delete keeps data safe but requires filtering in every query.

Remember to add is_deleted = FALSE in your SELECT queries to avoid showing deleted rows.

You can add an index on is_deleted for faster queries if the table is large.

Summary

Soft delete hides data by marking it instead of removing it.

Use a boolean column like is_deleted to track deleted rows.

Always filter queries to exclude soft deleted rows.

Practice

1. What is the main idea behind the soft delete pattern in databases?

easy

A. Marking data as deleted without actually removing it from the table

B. Permanently removing data from the database

C. Backing up data before deletion

D. Encrypting data before deletion

Solution

1. Step 1: Understand soft delete concept

   Soft delete means we do not remove data but mark it as deleted.

2. Step 2: Compare options

   Only Marking data as deleted without actually removing it from the table describes marking data instead of removing it.

3. Final Answer:

   Marking data as deleted without actually removing it from the table -> Option A

4. Quick Check:

   Soft delete = Mark data deleted, not remove [OK]

Hint: Soft delete means mark deleted, don't remove [OK]

Common Mistakes:

• Thinking soft delete removes data physically
• Confusing soft delete with backup
• Assuming encryption is part of soft delete

2. Which of the following SQL table columns is commonly used to implement a soft delete?

easy

A. deleted_at TIMESTAMP

B. is_deleted BOOLEAN

C. delete_flag VARCHAR(10)

D. remove_date DATE

Solution

1. Step 1: Identify common soft delete column

   The most common column is a boolean named is_deleted to mark true/false.

2. Step 2: Check other options

   Other columns can be used but is_deleted BOOLEAN is standard and simplest.

3. Final Answer:

   is_deleted BOOLEAN -> Option B

4. Quick Check:

   Soft delete uses boolean flag = is_deleted [OK]

Hint: Soft delete uses boolean flag like is_deleted [OK]

Common Mistakes:

• Using timestamp instead of boolean for soft delete
• Choosing vague column names
• Confusing soft delete with audit columns

3. Given the table users with a boolean column is_deleted, what will this query return?

SELECT * FROM users WHERE is_deleted = FALSE;

medium

A. Only users not marked as deleted

B. Only users marked as deleted

C. No users, syntax error

D. All users including deleted ones

Solution

1. Step 1: Understand the WHERE clause

   The query filters rows where is_deleted is FALSE, meaning not deleted.

2. Step 2: Determine returned rows

   Only users not marked deleted will be returned, excluding soft deleted rows.

3. Final Answer:

   Only users not marked as deleted -> Option A

4. Quick Check:

   WHERE is_deleted = FALSE returns active users [OK]

Hint: WHERE is_deleted = FALSE filters active rows [OK]

Common Mistakes:

• Thinking it returns deleted users
• Assuming syntax error due to boolean
• Confusing TRUE and FALSE values

4. You have this query to select active users:

SELECT * FROM users WHERE is_deleted = 0;

But it returns no rows even though some users are active. What is the likely problem?

medium

A. The table has no data

B. The is_deleted column is missing

C. The query syntax is invalid

D. The column is_deleted uses TRUE/FALSE, not 0/1

Solution

1. Step 1: Check data type of is_deleted

   If is_deleted is BOOLEAN, it expects TRUE/FALSE, not numeric 0/1.

2. Step 2: Understand why query returns no rows

   Using 0 may not match FALSE in some SQL dialects, causing no rows returned.

3. Final Answer:

   The column is_deleted uses TRUE/FALSE, not 0/1 -> Option D

4. Quick Check:

   Boolean columns need TRUE/FALSE, not 0/1 [OK]

Hint: Boolean needs TRUE/FALSE, not 0/1 [OK]

Common Mistakes:

• Assuming 0 equals FALSE in all SQL dialects
• Ignoring data type of column
• Thinking syntax error causes no rows

5. You want to implement soft delete on a products table. Which SQL statement correctly marks a product as deleted without removing it?

hard

A. INSERT INTO products (product_id, is_deleted) VALUES (10, TRUE);

B. DELETE FROM products WHERE product_id = 10;

C. UPDATE products SET is_deleted = TRUE WHERE product_id = 10;

D. SELECT * FROM products WHERE is_deleted = TRUE;

Solution

1. Step 1: Understand soft delete update

   Soft delete means updating a flag, not deleting the row.

2. Step 2: Analyze each option

   UPDATE products SET is_deleted = TRUE WHERE product_id = 10; updates is_deleted to TRUE for product 10, marking it deleted.

3. Final Answer:

   UPDATE products SET is_deleted = TRUE WHERE product_id = 10; -> Option C

4. Quick Check:

   Soft delete uses UPDATE to mark deleted [OK]

Hint: Use UPDATE to mark is_deleted TRUE [OK]

Common Mistakes:

• Using DELETE instead of UPDATE
• Trying to INSERT to mark deleted
• Using SELECT instead of modifying data