AUTO_INCREMENT vs SERIAL vs IDENTITY in SQL - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a MySQL table with an auto-incrementing primary key.

SQL

CREATE TABLE users (id INT [1] PRIMARY KEY, name VARCHAR(100));

Drag options to blanks, or click blank then click option'

AAUTO_INCREMENT

BSERIAL

CGENERATED

DIDENTITY

Attempts:

3 left

💡 Hint

Common Mistakes

Using SERIAL which is not valid in MySQL.

Using IDENTITY which is used in other databases like SQL Server.

In MySQL, AUTO_INCREMENT is used to create a column that automatically increases its value for each new row.

2fill in blank

medium

Complete the code to create a PostgreSQL table with a serial primary key.

SQL

CREATE TABLE products (product_id [1] PRIMARY KEY, product_name TEXT);

Drag options to blanks, or click blank then click option'

AAUTO_INCREMENT

BBIGSERIAL

CIDENTITY

DSERIAL

Attempts:

3 left

💡 Hint

Common Mistakes

Using AUTO_INCREMENT which is not valid in PostgreSQL.

Using IDENTITY which is a newer alternative but not SERIAL.

In PostgreSQL, SERIAL is a special data type that creates an auto-incrementing integer column.

3fill in blank

hard

Fix the error in the SQL Server table creation by completing the identity column syntax.

SQL

CREATE TABLE orders (order_id INT [1](1,1) PRIMARY KEY, order_date DATE);

Drag options to blanks, or click blank then click option'

AIDENTITY

BSERIAL

CAUTO_INCREMENT

DGENERATED

Attempts:

3 left

💡 Hint

Common Mistakes

Using AUTO_INCREMENT which is MySQL specific.

Using SERIAL which is PostgreSQL specific.

In SQL Server, IDENTITY(1,1) defines an auto-incrementing column starting at 1 and incrementing by 1.

4fill in blank

hard

Fill in the blank to create a PostgreSQL table using the IDENTITY syntax for auto-increment.

SQL

CREATE TABLE employees (emp_id INT GENERATED [1] AS IDENTITY PRIMARY KEY, emp_name TEXT);

Drag options to blanks, or click blank then click option'

ANOT NULL

BALWAYS

CBY DEFAULT

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using NOT NULL or NULL which are unrelated here.

PostgreSQL supports GENERATED BY DEFAULT AS IDENTITY to create auto-incrementing columns as a modern alternative to SERIAL.

5fill in blank

hard

Fill in the blanks to create a MySQL table with an auto-incrementing primary key and a default value.

SQL

CREATE TABLE sessions (session_id INT [1] PRIMARY KEY, user_id INT NOT NULL, status VARCHAR(20) DEFAULT [2]);

Drag options to blanks, or click blank then click option'

AAUTO_INCREMENT

B'active'

C'pending'

DSERIAL

Attempts:

3 left

💡 Hint

Common Mistakes

Using SERIAL in MySQL which is invalid.

Not quoting default string values.

In MySQL, AUTO_INCREMENT creates an auto-incrementing column. The DEFAULT keyword sets a default value for a column.

Practice

1. Which keyword is used in MySQL to automatically generate unique numbers for a primary key column?

easy

A. IDENTITY

B. SERIAL

C. SEQUENCE

D. AUTO_INCREMENT

Solution

1. Step 1: Understand MySQL auto-numbering

   MySQL uses AUTO_INCREMENT to generate unique numbers automatically for columns like primary keys.

2. Step 2: Compare with other keywords

   SERIAL is PostgreSQL-specific, IDENTITY is SQL standard used in SQL Server and newer PostgreSQL versions, and SEQUENCE is a separate object, not a column keyword.

3. Final Answer:

   AUTO_INCREMENT -> Option D

4. Quick Check:

   MySQL auto-number = AUTO_INCREMENT [OK]

Hint: MySQL uses AUTO_INCREMENT for auto numbering [OK]

Common Mistakes:

• Confusing SERIAL with MySQL syntax
• Using IDENTITY in MySQL
• Thinking SEQUENCE is a column keyword

2. Which of the following is the correct syntax to create an auto-incrementing primary key column in PostgreSQL using the SERIAL keyword?

easy

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

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

C. CREATE TABLE users (id IDENTITY PRIMARY KEY, name VARCHAR(50));

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

Solution

1. Step 1: Identify PostgreSQL SERIAL syntax

   In PostgreSQL, SERIAL is used as a shorthand to create an integer column that auto-increments.

2. Step 2: Check syntax correctness

   CREATE TABLE users (id SERIAL PRIMARY KEY, name VARCHAR(50)); uses id SERIAL PRIMARY KEY, which is valid. Options B and C use MySQL and SQL Server syntax incorrectly in PostgreSQL. CREATE TABLE users (id SEQUENCE PRIMARY KEY, name VARCHAR(50)); uses SEQUENCE incorrectly as a column type.

3. Final Answer:

   CREATE TABLE users (id SERIAL PRIMARY KEY, name VARCHAR(50)); -> Option A

4. Quick Check:

   SERIAL syntax in PostgreSQL = CREATE TABLE users (id SERIAL PRIMARY KEY, name VARCHAR(50)); [OK]

Hint: PostgreSQL uses SERIAL for auto-increment columns [OK]

Common Mistakes:

• Using AUTO_INCREMENT in PostgreSQL
• Using IDENTITY without parentheses
• Treating SEQUENCE as a column type

3. Given the following SQL Server table creation statement, what will be the output of inserting two rows without specifying the ID column?

CREATE TABLE products (id INT IDENTITY(1,1) PRIMARY KEY, name VARCHAR(20));
INSERT INTO products (name) VALUES ('Pen');
INSERT INTO products (name) VALUES ('Pencil');
SELECT * FROM products;

medium

A. [{"id": null, "name": "Pen"}, {"id": null, "name": "Pencil"}]

B. [{"id": 0, "name": "Pen"}, {"id": 1, "name": "Pencil"}]

C. [{"id": 1, "name": "Pen"}, {"id": 2, "name": "Pencil"}]

D. Syntax error on INSERT statements

Solution

1. Step 1: Understand IDENTITY behavior in SQL Server

   The IDENTITY(1,1) means the ID column starts at 1 and increments by 1 automatically for each new row.

2. Step 2: Predict inserted values

   Inserting two rows without specifying ID will assign IDs 1 and 2 respectively. The SELECT will show these IDs with names.

3. Final Answer:

   [{"id": 1, "name": "Pen"}, {"id": 2, "name": "Pencil"}] -> Option C

4. Quick Check:

   IDENTITY auto increments IDs starting at 1 [OK]

Hint: IDENTITY(1,1) starts numbering from 1 incrementing by 1 [OK]

Common Mistakes:

• Assuming ID starts at 0
• Expecting null IDs without insert values
• Thinking INSERT needs ID value explicitly

4. You try to create a table in PostgreSQL with this statement:

CREATE TABLE orders (order_id INT AUTO_INCREMENT PRIMARY KEY, order_date DATE);

What is the error and how to fix it?

medium

A. AUTO_INCREMENT is invalid in PostgreSQL; use SERIAL or IDENTITY instead

B. Missing parentheses after AUTO_INCREMENT

C. order_id must be VARCHAR for AUTO_INCREMENT

D. No error; statement is correct

Solution

1. Step 1: Identify AUTO_INCREMENT support

   PostgreSQL does not support AUTO_INCREMENT; this is a MySQL keyword.

2. Step 2: Correct PostgreSQL syntax

   Use SERIAL or GENERATED AS IDENTITY for auto-incrementing columns in PostgreSQL.

3. Final Answer:

   AUTO_INCREMENT is invalid in PostgreSQL; use SERIAL or IDENTITY instead -> Option A

4. Quick Check:

   PostgreSQL uses SERIAL or IDENTITY, not AUTO_INCREMENT [OK]

Hint: PostgreSQL rejects AUTO_INCREMENT; use SERIAL or IDENTITY [OK]

Common Mistakes:

• Trying to use MySQL syntax in PostgreSQL
• Assuming AUTO_INCREMENT works everywhere
• Ignoring error messages about syntax

5. You want to migrate a MySQL table with AUTO_INCREMENT primary key to PostgreSQL. Which approach correctly preserves auto-increment behavior?

hard

A. Keep AUTO_INCREMENT keyword as is in PostgreSQL

B. Replace AUTO_INCREMENT with SERIAL type for the primary key column

C. Use IDENTITY without specifying start value

D. Manually insert IDs for each row without auto-increment

Solution

1. Step 1: Understand MySQL to PostgreSQL migration

   MySQL uses AUTO_INCREMENT, but PostgreSQL does not support it directly.

2. Step 2: Use PostgreSQL equivalent

   PostgreSQL uses SERIAL as a shorthand for auto-increment integer columns, preserving behavior.

3. Final Answer:

   Replace AUTO_INCREMENT with SERIAL type for the primary key column -> Option B

4. Quick Check:

   MySQL AUTO_INCREMENT -> PostgreSQL SERIAL [OK]

Hint: Use SERIAL in PostgreSQL to replace MySQL AUTO_INCREMENT [OK]

Common Mistakes:

• Leaving AUTO_INCREMENT keyword unchanged
• Using IDENTITY without proper syntax
• Manually managing IDs instead of auto-increment