Soft delete pattern concept in SQL - Mini Project: Build & Apply

Implementing Soft Delete Pattern in SQL

📖 Scenario: You are managing a customer database for a small online store. Instead of permanently deleting customer records, you want to mark them as inactive so you can keep their data for future reference.

🎯 Goal: Build a simple SQL table with a soft delete pattern by adding an is_active column. Then write queries to mark customers as inactive and to select only active customers.

📋 What You'll Learn

Create a table called customers with columns id, name, and is_active

Set is_active to TRUE by default for new customers

Write an SQL query to mark a customer as inactive using UPDATE

Write an SQL query to select only active customers using WHERE is_active = TRUE

💡 Why This Matters

🌍 Real World

Soft delete is used in many applications to avoid losing data permanently and to allow easy recovery of deleted records.

💼 Career

Understanding soft delete patterns is important for database administrators and backend developers to manage data safely and comply with data retention policies.

1

Create the customers table

Write an SQL statement to create a table called customers with three columns: id as an integer primary key, name as text, and is_active as a boolean that defaults to TRUE.

SQL

-- Your code here to create the customers table

Hint

Use CREATE TABLE with the specified columns and set is_active default to TRUE.

2

Insert sample customers

Write SQL INSERT statements to add two customers to the customers table: one with id 1 and name 'Alice', and another with id 2 and name 'Bob'. Do not specify is_active so it uses the default.

SQL

CREATE TABLE customers (
    id INTEGER PRIMARY KEY,
    name TEXT,
    is_active BOOLEAN DEFAULT TRUE
);
-- Your code here to insert two customers

Hint

Use INSERT INTO customers (id, name) VALUES (...) twice with the exact values.

3

Mark a customer as inactive

Write an SQL UPDATE statement to set is_active to FALSE for the customer with id 2.

SQL

CREATE TABLE customers (
    id INTEGER PRIMARY KEY,
    name TEXT,
    is_active BOOLEAN DEFAULT TRUE
);

INSERT INTO customers (id, name) VALUES (1, 'Alice');
INSERT INTO customers (id, name) VALUES (2, 'Bob');
-- Your code here to mark Bob as inactive

Hint

Use UPDATE customers SET is_active = FALSE WHERE id = 2 to mark Bob inactive.

4

Select only active customers

Write an SQL SELECT statement to get all columns from customers where is_active is TRUE.

SQL

CREATE TABLE customers (
    id INTEGER PRIMARY KEY,
    name TEXT,
    is_active BOOLEAN DEFAULT TRUE
);

INSERT INTO customers (id, name) VALUES (1, 'Alice');
INSERT INTO customers (id, name) VALUES (2, 'Bob');

UPDATE customers SET is_active = FALSE WHERE id = 2;
-- Your code here to select active customers

Hint

Use SELECT * FROM customers WHERE is_active = TRUE to get only active customers.

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