Serial and identity columns in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

PostgreSQL

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

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

AINTEGER

BTEXT

CSERIAL

DBOOLEAN

Attempts:

3 left

💡 Hint

Common Mistakes

Using INTEGER without auto-increment will require manual value insertion.

Using TEXT or BOOLEAN types for primary keys is not appropriate.

The SERIAL keyword automatically creates an integer column that auto-increments with each new row, perfect for primary keys.

2fill in blank

medium

Complete the code to define an identity column that auto-generates values starting from 1.

PostgreSQL

CREATE TABLE orders (order_id INT GENERATED ALWAYS AS IDENTITY (START WITH [1]), order_date DATE);

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

A0

B100

CNULL

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Starting identity columns at 0 may cause confusion or errors.

Using NULL as a start value is invalid.

The START WITH 1 clause sets the first generated value to 1 for the identity column.

3fill in blank

hard

Fix the error in the code to correctly create a serial column named 'product_id'.

PostgreSQL

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

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

ASERIAL

BSERIAL PRIMARY

CSERIAL PRIMARY KEY

DINTEGER SERIAL

Attempts:

3 left

💡 Hint

Common Mistakes

Adding 'PRIMARY' inside the type causes syntax errors.

Using 'INTEGER SERIAL' is invalid syntax.

The correct syntax is to use SERIAL for the column type and specify PRIMARY KEY separately. The code already has PRIMARY KEY after the blank, so just SERIAL is needed.

4fill in blank

hard

Fill both blanks to create a table with an identity column that increments by 5 starting at 10.

PostgreSQL

CREATE TABLE invoices (invoice_id INT GENERATED [1] AS IDENTITY (START WITH [2], INCREMENT BY 5));

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

AALWAYS

BBY DEFAULT

C10

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'BY DEFAULT' when 'ALWAYS' is required.

Starting at 1 instead of 10.

GENERATED ALWAYS AS IDENTITY ensures the database always generates the value. START WITH 10 sets the first value to 10, and INCREMENT BY 5 increases by 5 each time.

5fill in blank

hard

Fill all three blanks to create a table with a serial column and a name column with a default value.

PostgreSQL

CREATE TABLE employees (emp_id [1] PRIMARY KEY, name VARCHAR(50) DEFAULT [2], hired_date DATE DEFAULT [3]);

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

ASERIAL

B'Unknown'

CCURRENT_DATE

DINTEGER

Attempts:

3 left

💡 Hint

Common Mistakes

Using INTEGER instead of SERIAL for auto-increment.

Missing quotes around default string values.

Using wrong default for date column.

SERIAL creates an auto-incrementing primary key. The name column defaults to the string 'Unknown' if no name is given. The hired_date column defaults to the current date.

Practice

1.

What is the main purpose of using SERIAL or IDENTITY columns in PostgreSQL?

easy

A. To automatically generate unique numbers for new rows

B. To store large text data efficiently

C. To create foreign key relationships

D. To improve query performance by indexing

Solution

1. Step 1: Understand the role of SERIAL and IDENTITY

   Both SERIAL and IDENTITY columns are designed to automatically generate unique numeric values for new rows in a table.

2. Step 2: Identify the main purpose

   This automatic numbering helps avoid manual entry and ensures unique identifiers for each row.

3. Final Answer:

   To automatically generate unique numbers for new rows -> Option A

4. Quick Check:

   Serial and identity columns = auto unique numbers [OK]

Hint: They auto-generate unique IDs for new rows [OK]

Common Mistakes:

• Thinking SERIAL stores text data
• Confusing SERIAL with indexing
• Assuming SERIAL creates foreign keys

2.

Which of the following is the correct syntax to create a table with an IDENTITY column named id in PostgreSQL?

CREATE TABLE users (
  id ???,
  name TEXT
);

easy

A. INT GENERATED AS IDENTITY

B. SERIAL PRIMARY KEY

C. INTEGER GENERATED ALWAYS AS IDENTITY PRIMARY KEY

D. INTEGER AUTO_INCREMENT PRIMARY KEY

Solution

1. Step 1: Recall IDENTITY syntax in PostgreSQL

   The correct syntax uses GENERATED ALWAYS AS IDENTITY for identity columns.

2. Step 2: Match the option with correct syntax

   INTEGER GENERATED ALWAYS AS IDENTITY PRIMARY KEY uses INTEGER GENERATED ALWAYS AS IDENTITY PRIMARY KEY, which is valid PostgreSQL syntax.

3. Final Answer:

   INTEGER GENERATED ALWAYS AS IDENTITY PRIMARY KEY -> Option C

4. Quick Check:

   IDENTITY syntax = GENERATED ALWAYS AS IDENTITY [OK]

Hint: IDENTITY uses GENERATED ALWAYS AS IDENTITY syntax [OK]

Common Mistakes:

• Using AUTO_INCREMENT (MySQL syntax)
• Confusing SERIAL with IDENTITY syntax
• Omitting GENERATED ALWAYS or BY DEFAULT

3.

Given the table creation:

CREATE TABLE products (
  product_id SERIAL PRIMARY KEY,
  name TEXT
);

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

SELECT product_id FROM products ORDER BY product_id;

What will be the output of the SELECT query?

medium

A. [3, 2, 1]

B. [0, 1, 2]

C. [NULL, NULL, NULL]

D. [1, 2, 3]

Solution

1. Step 1: Understand SERIAL behavior on insert

   The SERIAL column auto-increments starting from 1 for each inserted row.

2. Step 2: Check inserted rows and ordering

   Three rows inserted get product_id values 1, 2, and 3 respectively. Ordering by product_id returns them in ascending order.

3. Final Answer:

   [1, 2, 3] -> Option D

4. Quick Check:

   SERIAL starts at 1 and increments [OK]

Hint: SERIAL starts numbering at 1 by default [OK]

Common Mistakes:

• Assuming numbering starts at 0
• Expecting NULL values for SERIAL
• Thinking order is descending by default

4.

Consider this table creation:

CREATE TABLE orders (
  order_id INT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
  product TEXT
);

Which of the following statements will cause an error when inserting data?

medium

A. INSERT INTO orders (order_id, product) VALUES (5, 'Pen'), (5, 'Pencil');

B. INSERT INTO orders (order_id, product) VALUES (5, 'Pen');

C. INSERT INTO orders (product) VALUES ('Book');

D. INSERT INTO orders (order_id, product) VALUES (DEFAULT, 'Notebook');

Solution

1. Step 1: Understand GENERATED BY DEFAULT AS IDENTITY behavior

   This allows manual insertion of order_id or automatic generation if omitted.

2. Step 2: Analyze each insert statement

   INSERT INTO orders (order_id, product) VALUES (5, 'Pen'), (5, 'Pencil'); tries to insert two rows with the same order_id = 5, violating the primary key uniqueness constraint, causing an error.

3. Final Answer:

   INSERT with duplicate order_id values causes error -> Option A

4. Quick Check:

   Duplicate primary keys cause insert errors [OK]

Hint: Duplicate primary keys cause insert errors [OK]

Common Mistakes:

• Assuming GENERATED BY DEFAULT forbids manual IDs
• Ignoring primary key uniqueness
• Thinking DEFAULT keyword causes errors

5.

You want to create a table employees with a IDENTITY column named id that automatically generates unique IDs but also allows you to insert specific IDs manually when needed. Which is the best way to define the ID column?

hard

A. Use SERIAL PRIMARY KEY for the ID column

B. Use INTEGER GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY

C. Use INTEGER GENERATED ALWAYS AS IDENTITY PRIMARY KEY

D. Use INTEGER without any auto-generation

Solution

1. Step 1: Understand difference between GENERATED ALWAYS and BY DEFAULT

   GENERATED ALWAYS AS IDENTITY disallows manual inserts; BY DEFAULT allows manual ID insertion.

2. Step 2: Match requirement to syntax

   Since manual insertion is needed sometimes, GENERATED BY DEFAULT AS IDENTITY is the best choice.

3. Final Answer:

   INTEGER GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY -> Option B

4. Quick Check:

   BY DEFAULT allows manual IDs, ALWAYS does not [OK]

Hint: BY DEFAULT allows manual ID insertion [OK]

Common Mistakes:

• Choosing GENERATED ALWAYS which forbids manual IDs
• Using SERIAL which is less flexible
• Not using any auto-generation and risking duplicates