NOT NULL constraint behavior in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a table where the 'name' column cannot be empty.

SQL

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

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

ANULL

BNOT NULL

CDEFAULT NULL

DPRIMARY KEY

Attempts:

3 left

💡 Hint

Common Mistakes

Using NULL instead of NOT NULL allows empty values.

Using PRIMARY KEY when you only want to prevent nulls.

The NOT NULL constraint ensures that the 'name' column must have a value and cannot be left empty.

2fill in blank

medium

Complete the INSERT statement to add a user with a non-empty 'email' field that has a NOT NULL constraint.

SQL

INSERT INTO users (id, email) VALUES (1, [1]);

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

A'[e-mail]'

BDEFAULT

C''

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Inserting NULL into a NOT NULL column causes an error.

Using empty string '' may be allowed but is often not meaningful.

You must provide a valid non-null value for the 'email' column because it has a NOT NULL constraint. Using '[e-mail]' is correct.

3fill in blank

hard

Fix the error in the table creation by adding the correct constraint to the 'age' column to prevent NULL values.

SQL

CREATE TABLE people (id INT, age INT [1]);

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

AUNIQUE

BDEFAULT 0

CCHECK (age > 0)

DNOT NULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using DEFAULT 0 does not prevent NULL values.

CHECK constraints do not prevent NULL unless specified.

The NOT NULL constraint ensures the 'age' column cannot have NULL values, fixing the error.

4fill in blank

hard

Fill both blanks to create a table where 'username' cannot be NULL and has a default value 'guest'.

SQL

CREATE TABLE accounts (id INT, username VARCHAR(50) [1] [2]);

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

ANOT NULL

BDEFAULT 'guest'

CUNIQUE

DPRIMARY KEY

Attempts:

3 left

💡 Hint

Common Mistakes

Using UNIQUE instead of NOT NULL does not prevent NULL values.

Forgetting to set DEFAULT causes errors when no value is given.

The NOT NULL constraint prevents NULL values, and DEFAULT 'guest' sets a default username if none is provided.

5fill in blank

hard

Fill all three blanks to insert a new row with id 10, a non-null name, and a default age in the 'persons' table.

SQL

INSERT INTO persons (id, name, age) VALUES ([1], [2], [3]);

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

A10

B'Alice'

CDEFAULT

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using NULL for age when it has a NOT NULL constraint causes error.

Not quoting the name string causes syntax error.

We insert 10 for id, a non-null name 'Alice', and use DEFAULT for age to apply the default value.

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