NOT NULL constraint behavior in SQL - Mini Project: Build & Apply

Understanding NOT NULL Constraint Behavior in SQL

📖 Scenario: You are managing a small library database. You want to make sure that every book entry has a title and an author name. These fields should never be empty because they are essential for identifying books.

🎯 Goal: Create a table called Books with columns BookID, Title, and Author. Apply the NOT NULL constraint to Title and Author so that these fields cannot be left empty when inserting data.

📋 What You'll Learn

Create a table named Books with three columns: BookID, Title, and Author.

Set BookID as an integer primary key.

Apply the NOT NULL constraint to Title and Author columns.

Insert one valid book record with all fields filled.

Attempt to insert a book record with a NULL value in the Title column to see the constraint in action.

💡 Why This Matters

🌍 Real World

Ensuring important data fields are always filled in databases, like customer names or product IDs.

💼 Career

Database administrators and developers use NOT NULL constraints to maintain data quality and prevent errors.

1

Create the Books table with columns

Write a SQL statement to create a table called Books with three columns: BookID as an integer primary key, Title as text, and Author as text.

SQL

-- Create the Books table with BookID, Title, and Author columns
-- Your code here

Hint

Use CREATE TABLE Books and define BookID as INTEGER PRIMARY KEY. Add Title and Author as TEXT columns.

2

Add NOT NULL constraints to Title and Author

Modify the Books table creation SQL to add the NOT NULL constraint to the Title and Author columns.

SQL

CREATE TABLE Books (
    BookID INTEGER PRIMARY KEY,
    Title TEXT NOT NULL,
    Author TEXT NOT NULL
);
-- Your code here

Hint

Add NOT NULL after the TEXT type for both Title and Author columns.

3

Insert a valid book record

Write a SQL INSERT statement to add a book with BookID 1, Title 'The Great Gatsby', and Author 'F. Scott Fitzgerald' into the Books table.

SQL

CREATE TABLE Books (
    BookID INTEGER PRIMARY KEY,
    Title TEXT NOT NULL,
    Author TEXT NOT NULL
);

-- Insert a valid book record
-- Your code here

Hint

Use INSERT INTO Books (BookID, Title, Author) VALUES (1, 'The Great Gatsby', 'F. Scott Fitzgerald');

4

Attempt to insert a book with NULL Title

Write a SQL INSERT statement to add a book with BookID 2, Title as NULL, and Author 'Unknown Author' into the Books table. This will demonstrate the NOT NULL constraint behavior.

SQL

CREATE TABLE Books (
    BookID INTEGER PRIMARY KEY,
    Title TEXT NOT NULL,
    Author TEXT NOT NULL
);

INSERT INTO Books (BookID, Title, Author) VALUES (1, 'The Great Gatsby', 'F. Scott Fitzgerald');

-- Attempt to insert a book with NULL Title
-- Your code here

Hint

Use INSERT INTO Books (BookID, Title, Author) VALUES (2, NULL, 'Unknown Author'); to test the constraint.

Practice

1. What does the NOT NULL constraint do in a database table?

easy

A. It allows a column to have duplicate values.

B. It ensures that a column cannot have empty or missing values.

C. It automatically fills the column with default values.

D. It deletes rows with null values in any column.

Solution

1. Step 1: Understand the purpose of NOT NULL

   The NOT NULL constraint prevents a column from having NULL (empty) values, ensuring data completeness.

2. Step 2: Compare options with the definition

   Only the description 'It ensures that a column cannot have empty or missing values.' correctly describes this behavior; others describe different constraints or actions.

3. Final Answer:

   It ensures that a column cannot have empty or missing values. -> Option B

4. Quick Check:

   NOT NULL = no empty values [OK]

Hint: NOT NULL means no empty values allowed in that column [OK]

Common Mistakes:

• Thinking NOT NULL allows duplicates
• Confusing NOT NULL with default values
• Assuming NOT NULL deletes rows

2. Which of the following is the correct syntax to add a NOT NULL constraint to a column named email in a table users?

easy

A. ALTER TABLE users MODIFY email NOT NULL;

B. ALTER TABLE users ADD email NOT NULL;

C. ALTER TABLE users SET email NOT NULL;

D. ALTER TABLE users CHANGE email TO NOT NULL;

Solution

1. Step 1: Recall ALTER TABLE syntax for modifying column constraints

   To add NOT NULL, we use ALTER TABLE with MODIFY (MySQL syntax).

2. Step 2: Evaluate each option's syntax

   ALTER TABLE users MODIFY email NOT NULL; uses correct syntax; others use invalid or incorrect commands.

3. Final Answer:

   ALTER TABLE users MODIFY email NOT NULL; -> Option A

4. Quick Check:

   ALTER TABLE + MODIFY + NOT NULL = correct syntax [OK]

Hint: Use ALTER TABLE with MODIFY to change NOT NULL [OK]

Common Mistakes:

• Using ADD instead of MODIFY for existing columns
• Using SET or CHANGE incorrectly
• Missing semicolon or wrong keywords

3. Given the table products with a price column defined as DECIMAL NOT NULL, what happens when you run this query?

INSERT INTO products (name, price) VALUES ('Pen', NULL);

medium

A. The query fails with an error due to NOT NULL violation.

B. The row is inserted with price set to 0.

C. The row is inserted with price set to NULL anyway.

D. The query succeeds but price is ignored.

Solution

1. Step 1: Understand NOT NULL effect on insert

   NOT NULL means the column cannot accept NULL values during insert or update.

2. Step 2: Analyze the insert statement

   Inserting NULL into a NOT NULL column causes the database to reject the query with an error.

3. Final Answer:

   The query fails with an error due to NOT NULL violation. -> Option A

4. Quick Check:

   Inserting NULL into NOT NULL column = error [OK]

Hint: Inserting NULL into NOT NULL column causes error [OK]

Common Mistakes:

• Assuming NULL becomes 0 automatically
• Thinking NULL is allowed silently
• Believing the query will partially succeed

4. You have this table creation statement:

CREATE TABLE employees (id INT, name VARCHAR(50) NOT NULL);

You try to insert:

INSERT INTO employees (id, name) VALUES (1, NULL);

But it gives an error. What is the fix?

medium

A. Add DEFAULT NULL to the name column.

B. Change the id column to NOT NULL.

C. Remove NOT NULL from the name column.

D. Insert a non-NULL value for name.

Solution

1. Step 1: Identify cause of error

   The name column is NOT NULL, so inserting NULL causes an error.

2. Step 2: Find correct fix

   To fix, insert a valid non-NULL value for name; changing other columns or constraints won't help here.

3. Final Answer:

   Insert a non-NULL value for name. -> Option D

4. Quick Check:

   NOT NULL column must get non-NULL value [OK]

Hint: Insert real value for NOT NULL columns to avoid errors [OK]

Common Mistakes:

• Removing NOT NULL without reason
• Changing unrelated columns
• Adding DEFAULT NULL which conflicts with NOT NULL

5. You want to ensure that the username column in your accounts table always has a value and cannot be empty. Which of these combined approaches is best to enforce this?

CREATE TABLE accounts (
  id INT PRIMARY KEY,
  username VARCHAR(30) NOT NULL CHECK (username <> '')
);

hard

A. Use NOT NULL only, because it blocks empty strings too.

B. Use CHECK only, because NOT NULL allows empty strings.

C. Use both NOT NULL and CHECK to block NULL and empty strings.

D. No constraints needed; application code will handle it.

Solution

1. Step 1: Understand NOT NULL and empty strings

   NOT NULL prevents NULL values but allows empty strings ('').

2. Step 2: Understand CHECK constraint role

   CHECK (username <> '') prevents empty strings but allows NULL unless combined with NOT NULL.

3. Step 3: Combine constraints for full validation

   Using both NOT NULL and CHECK ensures username is neither NULL nor empty.

4. Final Answer:

   Use both NOT NULL and CHECK to block NULL and empty strings. -> Option C

5. Quick Check:

   NOT NULL + CHECK = no NULL or empty [OK]

Hint: Combine NOT NULL and CHECK to block NULL and empty strings [OK]

Common Mistakes:

• Assuming NOT NULL blocks empty strings
• Using CHECK without NOT NULL allows NULLs
• Relying only on application code