AUTO_INCREMENT behavior in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this AUTO_INCREMENT insert sequence?

Consider a table users with an id column set as AUTO_INCREMENT starting at 1. After inserting three rows, the id values are 1, 2, and 3. If we delete the row with id = 2 and then insert a new row, what will be the id of the new row?

SQL

CREATE TABLE users (id INT AUTO_INCREMENT PRIMARY KEY, name VARCHAR(50));
INSERT INTO users (name) VALUES ('Alice'), ('Bob'), ('Charlie');
DELETE FROM users WHERE id = 2;
INSERT INTO users (name) VALUES ('David');
SELECT id, name FROM users ORDER BY id;

A

1, Alice
2, Bob
3, Charlie
4, David

B

1, Alice
3, Charlie
4, David

C

1, Alice
2, David
3, Charlie

D

1, Alice
3, Charlie
2, David

💡 Hint

AUTO_INCREMENT does not reuse deleted IDs by default.

When a row with an AUTO_INCREMENT ID is deleted, the next inserted row gets a new ID incremented from the highest existing ID. It does not reuse the deleted ID.

🧠 Conceptual

intermediate

How does AUTO_INCREMENT behave after a server restart?

If a MySQL server is restarted, what happens to the AUTO_INCREMENT counter for a table that already has rows?

A The AUTO_INCREMENT counter continues from the highest existing ID plus one.

B The AUTO_INCREMENT counter starts from the last inserted ID, even if rows were deleted.

C The AUTO_INCREMENT counter is lost and must be manually reset.

D The AUTO_INCREMENT counter resets to 1 regardless of existing rows.

💡 Hint

Think about how the server remembers the last used ID.

On restart, MySQL reads the highest existing AUTO_INCREMENT value from the table and continues from there to avoid duplicate IDs.

📝 Syntax

advanced

Which statement correctly creates a table with AUTO_INCREMENT starting at 1000?

You want to create a table orders with an order_id column that auto-increments starting from 1000. Which SQL statement is correct?

A CREATE TABLE orders (order_id INT AUTO_INCREMENT PRIMARY KEY START WITH 1000);

B CREATE TABLE orders (order_id INT PRIMARY KEY AUTO_INCREMENT=1000);

C CREATE TABLE orders (order_id INT AUTO_INCREMENT PRIMARY KEY) AUTO_INCREMENT=1000;

D CREATE TABLE orders (order_id INT AUTO_INCREMENT PRIMARY KEY) START=1000;

💡 Hint

The AUTO_INCREMENT starting value is set outside the column definition.

In MySQL, the starting value for AUTO_INCREMENT is set as a table option after the column definitions using AUTO_INCREMENT=number.

🔧 Debug

advanced

Why does this insert fail with duplicate key error?

Given the table products with product_id as AUTO_INCREMENT primary key, the following insert causes a duplicate key error. Why? INSERT INTO products (product_id, name) VALUES (1, 'Pen');

SQL

CREATE TABLE products (product_id INT AUTO_INCREMENT PRIMARY KEY, name VARCHAR(50));
INSERT INTO products (name) VALUES ('Pencil');
INSERT INTO products (product_id, name) VALUES (1, 'Pen');

A Because the name 'Pen' is a duplicate and violates a unique constraint.

B Because AUTO_INCREMENT columns cannot be explicitly assigned values.

C Because the table does not allow inserting any rows after the first.

D Because the value 1 for product_id already exists from the first insert.

💡 Hint

Check the existing IDs before the insert.

The first insert auto-assigned product_id = 1. Trying to insert another row with product_id = 1 causes a duplicate primary key error.

❓ optimization

expert

How to optimize bulk inserts with AUTO_INCREMENT to avoid gaps?

You need to insert 10,000 rows into a table with an AUTO_INCREMENT primary key. You want to minimize gaps in the IDs caused by transaction rollbacks or deletes. Which approach helps achieve this?

A Disable autocommit and insert all rows in a single transaction, then commit once.

B Insert rows one by one with autocommit enabled to ensure each ID is assigned sequentially.

C Manually assign IDs instead of using AUTO_INCREMENT to control gaps.

D Delete rows after insert to fill gaps in AUTO_INCREMENT IDs.

💡 Hint

Think about how transactions affect AUTO_INCREMENT values.

Using a single transaction for bulk inserts avoids gaps caused by rollbacks because IDs are only assigned once the transaction commits.

Practice

1. What does the AUTO_INCREMENT attribute do in a SQL table?

easy

A. Encrypts the data in the column

B. Deletes duplicate rows automatically

C. Automatically assigns a unique number to new rows

D. Prevents any new rows from being added

Solution

1. Step 1: Understand the purpose of AUTO_INCREMENT

   AUTO_INCREMENT is used to generate a unique number for each new row automatically, usually for primary keys.

2. Step 2: Compare options with this behavior

   Only Automatically assigns a unique number to new rows describes this behavior correctly; others describe unrelated actions.

3. Final Answer:

   Automatically assigns a unique number to new rows -> Option C

4. Quick Check:

   AUTO_INCREMENT = unique number assignment [OK]

Hint: AUTO_INCREMENT means unique numbers auto-assigned [OK]

Common Mistakes:

• Thinking AUTO_INCREMENT deletes duplicates
• Confusing AUTO_INCREMENT with encryption
• Assuming AUTO_INCREMENT blocks inserts

2. Which of the following is the correct syntax to create a table with an AUTO_INCREMENT column in SQL?

easy

A. CREATE TABLE users (id INT INCREMENT_AUTO, name VARCHAR(50), PRIMARY KEY(id));

B. CREATE TABLE users (id INT AUTO_INCREMENT, name VARCHAR(50), PRIMARY KEY(id));

C. CREATE TABLE users (id INT AUTO_INCREMENT PRIMARY, name VARCHAR(50));

D. CREATE TABLE users (id INT AUTO_INCREMENTED, name VARCHAR(50), PRIMARY KEY(id));

Solution

1. Step 1: Review correct AUTO_INCREMENT syntax

   The correct syntax is to declare the column as INT AUTO_INCREMENT and set it as PRIMARY KEY.

2. Step 2: Check each option for syntax errors

   CREATE TABLE users (id INT AUTO_INCREMENT, name VARCHAR(50), PRIMARY KEY(id)); matches the correct syntax. Options B, C, and D use invalid keywords or misplaced PRIMARY KEY.

3. Final Answer:

   CREATE TABLE users (id INT AUTO_INCREMENT, name VARCHAR(50), PRIMARY KEY(id)); -> Option B

4. Quick Check:

   Correct AUTO_INCREMENT syntax = CREATE TABLE users (id INT AUTO_INCREMENT, name VARCHAR(50), PRIMARY KEY(id)); [OK]

Hint: AUTO_INCREMENT goes with INT and PRIMARY KEY [OK]

Common Mistakes:

• Using wrong keywords like INCREMENT_AUTO
• Placing PRIMARY KEY inside column definition incorrectly
• Adding suffixes like AUTO_INCREMENTED

3. Given the table products with id INT AUTO_INCREMENT PRIMARY KEY and name VARCHAR(50), what will be the id values after these inserts?

INSERT INTO products (name) VALUES ('Pen');
INSERT INTO products (name) VALUES ('Pencil');
INSERT INTO products (name) VALUES ('Eraser');

medium

A. [1, 2, 3]

B. [0, 1, 2]

C. [NULL, NULL, NULL]

D. [1, 1, 1]

Solution

1. Step 1: Understand AUTO_INCREMENT numbering

   Each insert without specifying id gets the next unique number starting from 1 by default.

2. Step 2: Apply to the three inserts

   First insert gets 1, second gets 2, third gets 3 as id values.

3. Final Answer:

   [1, 2, 3] -> Option A

4. Quick Check:

   Sequential AUTO_INCREMENT IDs = [1, 2, 3] [OK]

Hint: AUTO_INCREMENT starts at 1 and increments by 1 [OK]

Common Mistakes:

• Assuming IDs start at 0
• Expecting NULL values for AUTO_INCREMENT
• Thinking IDs repeat for each insert

4. You have a table orders with order_id INT AUTO_INCREMENT PRIMARY KEY. You try to insert a row with order_id = 0 manually, but get an error. What is the likely cause?

medium

A. You cannot insert a value manually into an AUTO_INCREMENT column without special syntax

B. The table does not allow any inserts

C. The column is not set as PRIMARY KEY

D. The AUTO_INCREMENT value must be a string

Solution

1. Step 1: Understand manual inserts on AUTO_INCREMENT columns

   By default, inserting a value manually into an AUTO_INCREMENT column can cause errors unless allowed explicitly.

2. Step 2: Analyze the error cause

   The error likely occurs because the database expects to manage the AUTO_INCREMENT values automatically, and manual insertion conflicts with this.

3. Final Answer:

   You cannot insert a value manually into an AUTO_INCREMENT column without special syntax -> Option A

4. Quick Check:

   Manual insert on AUTO_INCREMENT needs care = You cannot insert a value manually into an AUTO_INCREMENT column without special syntax [OK]

Hint: AUTO_INCREMENT columns usually reject manual values [OK]

Common Mistakes:

• Assuming inserts are blocked entirely
• Confusing PRIMARY KEY with AUTO_INCREMENT behavior
• Thinking AUTO_INCREMENT accepts strings

5. A table employees has emp_id INT AUTO_INCREMENT PRIMARY KEY. After deleting the row with emp_id = 3, you insert a new row without specifying emp_id. What will be the emp_id of the new row?

hard

A. 3

B. NULL

C. 1

D. The next highest number after the last inserted ID

Solution

1. Step 1: Understand AUTO_INCREMENT behavior after deletions

   Deleting a row does not reset or reuse the deleted AUTO_INCREMENT value by default.

2. Step 2: Determine the new ID assigned

   The new row gets the next number after the highest previously assigned ID, not the deleted one.

3. Final Answer:

   The next highest number after the last inserted ID -> Option D

4. Quick Check:

   Deleted IDs are not reused automatically = The next highest number after the last inserted ID [OK]

Hint: AUTO_INCREMENT never reuses deleted IDs by default [OK]

Common Mistakes:

• Assuming deleted IDs get reused automatically
• Thinking new inserts reset to 1
• Expecting NULL for AUTO_INCREMENT columns