NOT NULL and DEFAULT constraints in MySQL - Step-by-Step Execution

Concept Flow - NOT NULL and DEFAULT constraints

Create Table

↓

Define Column

↓

Apply NOT NULL?

Yes→Reject NULL inserts

No↓

Apply DEFAULT?

Yes→Use default if no value given

No↓

Insert Row

↓

Check Constraints

↓

Accept or Reject Row

When creating a table, columns can be set NOT NULL to reject missing values, and DEFAULT to supply a value if none is given during insert.

Execution Sample

MySQL

CREATE TABLE users (
  id INT NOT NULL,
  name VARCHAR(20) NOT NULL DEFAULT 'guest'
);

INSERT INTO users (id) VALUES (1);

Creates a table with NOT NULL and DEFAULT constraints, then inserts a row with only id, name uses default.

Execution Table

Step	Action	Column	Value Provided	Constraint Check	Resulting Value	Row Inserted?
1	Start Insert	id	1	NOT NULL check passes	1	Pending
2	Start Insert	name	NULL (not provided)	NOT NULL check passes (value missing but DEFAULT exists)	Check DEFAULT	Pending
3	Apply DEFAULT	name	NULL	DEFAULT 'guest' applied	'guest'	Pending
4	Final Check	all columns	id=1, name='guest'	All constraints satisfied	id=1, name='guest'	Yes

💡 Insert succeeds because name uses DEFAULT 'guest' and id is provided and NOT NULL.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
id	NULL	1	1	1	1
name	NULL	NULL	NULL	'guest'	'guest'

Key Moments - 2 Insights

Why does the insert not fail when 'name' is not provided even though it is NOT NULL?

Because the DEFAULT constraint provides 'guest' as the value for 'name' when no value is given, as shown in execution_table row 3.

What happens if we try to insert NULL explicitly into a NOT NULL column without DEFAULT?

The insert fails immediately at the NOT NULL check, rejecting the row. This is implied by the constraint check in execution_table row 2.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what value is assigned to 'name'?

A Empty string

B NULL

C 'guest'

D No value assigned

💡 Hint

Check the 'Resulting Value' column in execution_table row 3.

At which step does the NOT NULL constraint get checked for 'name'?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Constraint Check' column in execution_table row 2.

If the DEFAULT constraint was removed from 'name', what would happen when inserting without 'name'?

A Insert fails due to NOT NULL violation

B Insert succeeds with NULL in 'name'

C Insert succeeds with empty string in 'name'

D Insert succeeds with 'guest' anyway

💡 Hint

Refer to key_moments explanation about NOT NULL without DEFAULT.

Concept Snapshot

NOT NULL prevents missing values in a column.
DEFAULT sets a value if none is given during insert.
If NOT NULL and DEFAULT both exist, DEFAULT fills missing values.
Inserts fail if NOT NULL column has no value and no DEFAULT.
Use these constraints to ensure data integrity.

Full Transcript

This visual execution shows how MySQL handles NOT NULL and DEFAULT constraints during an insert. When inserting a row, each column is checked. If a NOT NULL column is missing a value, the database rejects the insert unless a DEFAULT value is defined. In the example, the 'name' column is NOT NULL with a DEFAULT of 'guest'. When inserting only 'id', the database applies the default 'guest' to 'name' and accepts the row. The execution table traces each step: checking provided values, applying constraints, and final acceptance. This helps beginners see how constraints protect data and how defaults help avoid insert errors.

Practice

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

easy

A. It automatically fills the column with a default value.

B. It deletes rows where the column is empty.

C. It allows the column to accept any type of data.

D. It ensures the column cannot have empty or missing values.

Solution

1. Step 1: Understand NOT NULL constraint

   The NOT NULL constraint means a column must always have a value; it cannot be left empty or null.

2. Step 2: Compare with other options

   Only It ensures the column cannot have empty or missing values. correctly describes this behavior. It automatically fills the column with a default value. describes DEFAULT, C is unrelated, and D is incorrect.

3. Final Answer:

   It ensures the column cannot have empty or missing values. -> Option D

4. Quick Check:

   NOT NULL means no empty values allowed [OK]

Hint: NOT NULL means value required, no blanks allowed [OK]

Common Mistakes:

• Confusing NOT NULL with DEFAULT
• Thinking NOT NULL deletes rows
• Assuming NOT NULL allows empty strings

2. Which of the following is the correct syntax to add a NOT NULL and DEFAULT constraint to a column named age of type INT in MySQL?

easy

A. ALTER TABLE users MODIFY age INT DEFAULT 18 NULL;

B. ALTER TABLE users MODIFY age INT NOT NULL DEFAULT 18;

C. ALTER TABLE users CHANGE age INT DEFAULT 18 NOT NULL;

D. ALTER TABLE users ADD age INT DEFAULT NOT NULL 18;

Solution

1. Step 1: Review correct ALTER TABLE syntax

   To modify a column with NOT NULL and DEFAULT, the syntax is: MODIFY column_name data_type NOT NULL DEFAULT value.

2. Step 2: Check each option

   ALTER TABLE users MODIFY age INT NOT NULL DEFAULT 18; matches this syntax exactly. Options B and C have wrong order or keywords. ALTER TABLE users MODIFY age INT DEFAULT 18 NULL; allows NULL, which is incorrect.

3. Final Answer:

   ALTER TABLE users MODIFY age INT NOT NULL DEFAULT 18; -> Option B

4. Quick Check:

   Correct syntax uses MODIFY with NOT NULL then DEFAULT [OK]

Hint: Use MODIFY column TYPE NOT NULL DEFAULT value [OK]

Common Mistakes:

• Placing DEFAULT before NOT NULL
• Using ADD instead of MODIFY for existing columns
• Allowing NULL when NOT NULL is required

3. Given the table employees with column status VARCHAR(10) NOT NULL DEFAULT 'active', what will be the value of status after this insert?

INSERT INTO employees (name) VALUES ('John');

medium

A. Empty string ''

B. NULL

C. 'active'

D. Error due to missing status

Solution

1. Step 1: Understand DEFAULT and NOT NULL behavior

   The column status cannot be NULL and has a default value 'active'. If no value is given, MySQL uses the default.

2. Step 2: Analyze the INSERT statement

   The insert only provides name, so status uses its default 'active'. No error occurs.

3. Final Answer:

   'active' -> Option C

4. Quick Check:

   Missing value uses DEFAULT when NOT NULL [OK]

Hint: Missing NOT NULL column uses DEFAULT value automatically [OK]

Common Mistakes:

• Expecting NULL instead of default
• Thinking insert will fail without explicit value
• Assuming empty string is default

4. You have this table creation statement:

CREATE TABLE products (id INT NOT NULL, price DECIMAL(5,2) DEFAULT 0.00);

What is the problem with this table definition?

medium

A. The id column is NOT NULL but has no DEFAULT, causing insert errors.

B. The id column allows NULL values.

C. The price column cannot have a default value.

D. There is no PRIMARY KEY defined, so the table is invalid.

Solution

1. Step 1: Check NOT NULL columns without DEFAULT

   The id column is NOT NULL but has no default value. If an insert omits id, it will cause an error.

2. Step 2: Analyze other options

   The price column cannot have a default value. is wrong because price can have a default. The id column allows NULL values. is wrong because id is NOT NULL. There is no PRIMARY KEY defined, so the table is invalid. is not mandatory for table creation.

3. Final Answer:

   The id column is NOT NULL but has no DEFAULT, causing insert errors. -> Option A

4. Quick Check:

   NOT NULL without DEFAULT requires explicit value on insert [OK]

Hint: NOT NULL columns need value or DEFAULT to avoid insert errors [OK]

Common Mistakes:

• Assuming NOT NULL columns get default NULL
• Thinking PRIMARY KEY is required for table creation
• Believing DEFAULT is mandatory for NOT NULL

5. You want to create a table orders where the status column must never be empty and should default to 'pending'. Which of these statements correctly creates this table?

hard

A. CREATE TABLE orders (id INT NOT NULL, status VARCHAR(20) NOT NULL DEFAULT 'pending');

B. CREATE TABLE orders (id INT NOT NULL, status VARCHAR(20) DEFAULT 'pending');

C. CREATE TABLE orders (id INT NOT NULL, status VARCHAR(20) NOT NULL);

D. CREATE TABLE orders (id INT NOT NULL, status VARCHAR(20) DEFAULT NOT NULL 'pending');

Solution

1. Step 1: Identify constraints needed

   Status must never be empty (NOT NULL) and default to 'pending' if no value is given.

2. Step 2: Check each CREATE TABLE option

   CREATE TABLE orders (id INT NOT NULL, status VARCHAR(20) NOT NULL DEFAULT 'pending'); correctly uses NOT NULL and DEFAULT together. CREATE TABLE orders (id INT NOT NULL, status VARCHAR(20) DEFAULT 'pending'); misses NOT NULL, allowing NULLs. CREATE TABLE orders (id INT NOT NULL, status VARCHAR(20) NOT NULL); misses DEFAULT, so no automatic value. CREATE TABLE orders (id INT NOT NULL, status VARCHAR(20) DEFAULT NOT NULL 'pending'); has wrong syntax.

3. Final Answer:

   CREATE TABLE orders (id INT NOT NULL, status VARCHAR(20) NOT NULL DEFAULT 'pending'); -> Option A

4. Quick Check:

   NOT NULL with DEFAULT sets required and default value [OK]

Hint: Use NOT NULL before DEFAULT for required columns [OK]

Common Mistakes:

• Omitting NOT NULL allows NULL values
• Placing DEFAULT before NOT NULL
• Using invalid syntax with DEFAULT NOT NULL