AUTO_INCREMENT behavior in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

SQL

CREATE TABLE users (id INT [1] PRIMARY KEY, name VARCHAR(50));

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

AVARCHAR(10)

BAUTO_INCREMENT

CINTEGER

DPRIMARY KEY

Attempts:

3 left

💡 Hint

Common Mistakes

Using just INTEGER without AUTO_INCREMENT.

Putting PRIMARY KEY twice.

Using VARCHAR for an auto-incrementing number.

The AUTO_INCREMENT keyword makes the id column automatically increase with each new row.

2fill in blank

medium

Complete the code to insert a new user without specifying the auto-increment column.

SQL

INSERT INTO users (name) VALUES ([1]);

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

ANULL

B1

C'Alice'

D'id'

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to insert a value for the auto-increment column.

Using NULL instead of a name string.

Inserting the column name instead of a value.

When inserting, you only provide the name. The id is generated automatically.

3fill in blank

hard

Fix the error in the code to reset the auto-increment counter to 1.

SQL

ALTER TABLE users [1] = 1;

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

AAUTO_INCREMENT

BRESTART

CINCREMENT

DRESET

Attempts:

3 left

💡 Hint

Common Mistakes

Using RESET or RESTART which are not valid keywords here.

Trying to use INCREMENT instead of AUTO_INCREMENT.

The correct syntax to reset the auto-increment value is to set AUTO_INCREMENT to 1.

4fill in blank

hard

Fill both blanks to create a table where the auto-increment starts at 1000.

SQL

CREATE TABLE orders (order_id INT [1] PRIMARY KEY) [2];

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

AAUTO_INCREMENT

BSTART WITH 1000

CAUTO_INCREMENT=1000

DINCREMENT BY 10

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'START WITH' or 'INCREMENT BY 10' which are not valid in this SQL dialect.

Putting the starting value inside the column definition.

The AUTO_INCREMENT keyword defines the column, and AUTO_INCREMENT=1000 sets the starting value for the table.

5fill in blank

hard

Fill in the blanks to select the next auto-increment value, insert a new row, and then get the last inserted id.

SQL

SELECT [1] FROM information_schema.tables WHERE table_name = 'users';
INSERT INTO users (name) VALUES ('Bob');
SELECT [2]();

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

AAUTO_INCREMENT

BLAST_INSERT_ID

DMAX(id)

Attempts:

3 left

💡 Hint

Common Mistakes

Using MAX(id) instead of LAST_INSERT_ID() to get the last inserted id.

Confusing AUTO_INCREMENT keyword with function names.

The first query gets the next auto-increment value. The last query gets the id of the last inserted row using LAST_INSERT_ID().

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