NOT NULL constraint behavior in SQL - Step-by-Step Execution

Concept Flow - NOT NULL constraint behavior

Start Insert/Update

↓

Check Column Value

↓

Is Value NULL?

No→Allow Operation

Yes↓

Reject Operation with Error

↓

End

When inserting or updating, the database checks if a NOT NULL column has a NULL value. If yes, it rejects the operation with an error; otherwise, it allows it.

Execution Sample

SQL

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

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

This code tries to insert a row with a NULL value in the 'name' column, which has a NOT NULL constraint.

Execution Table

Step	Action	Column Checked	Value	Result	Notes
1	Start INSERT	id	1	OK	Value is not NULL, passes NOT NULL check
2	Check 'name' column	name	NULL	Error	NULL value violates NOT NULL constraint
3	Reject INSERT	-	-	Failed	Insert operation aborted due to NOT NULL violation

💡 Insert fails because 'name' column value is NULL but has NOT NULL constraint

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
id	undefined	1	1	1
name	undefined	NULL	NULL	NULL
Insert Operation Status	pending	pending	error	failed

Key Moments - 2 Insights

Why does the insert fail even though only one column has NULL?

The NOT NULL constraint applies to each column individually. In the execution_table row 2, the 'name' column is checked and found NULL, which violates its NOT NULL constraint, causing the entire insert to fail.

Can other columns without NOT NULL constraints accept NULL values?

Yes. Only columns defined with NOT NULL reject NULL values. Columns without this constraint can accept NULL. This is shown in the execution_table where 'id' with NOT NULL passes with value 1, but 'name' fails with NULL.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result when checking the 'id' column?

A Error due to NULL value

B OK, value is not NULL

C Skipped check

D Warning but allowed

💡 Hint

Refer to execution_table row 1 under 'Result' column

At which step does the insert operation get rejected?

A Step 1

B Step 2

C Step 3

D After all steps

💡 Hint

Look at execution_table row 3 under 'Result' and 'Notes'

If the 'name' column did not have NOT NULL constraint, what would happen at step 2?

A Insert would succeed

B Insert would still fail

C Error would change to warning

D Database would ignore the column

💡 Hint

Consider the role of NOT NULL constraint in execution_table row 2

Concept Snapshot

NOT NULL constraint ensures a column cannot have NULL values.
When inserting or updating, if a NOT NULL column receives NULL, the operation fails.
Each NOT NULL column is checked individually.
Columns without NOT NULL allow NULL values.
Violations cause immediate rejection with an error.

Full Transcript

The NOT NULL constraint in SQL prevents columns from having NULL values. When you insert or update data, the database checks each NOT NULL column to ensure the value is not NULL. If it finds a NULL value in such a column, it rejects the operation with an error. For example, inserting a row with NULL in a NOT NULL column causes the insert to fail. Columns without NOT NULL constraints can accept NULL values without errors. This behavior helps keep important data always present in the database.

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