DEFAULT values in SQL - Step-by-Step Execution

Concept Flow - DEFAULT values

Start Insert

↓

Check each column

↓

Is value provided?

Yes→Use provided value

No↓

Is DEFAULT defined?

Yes→Use DEFAULT value

No↓

Use NULL or error if NOT NULL

↓

Insert row with final values

↓

End

When inserting data, SQL checks each column. If no value is given, it uses the DEFAULT if set, else NULL or error.

Execution Sample

SQL

CREATE TABLE users (
  id INT,
  name VARCHAR(20) DEFAULT 'Guest',
  age INT DEFAULT 18
);

INSERT INTO users (id) VALUES (1);

Creates a table with default values for name and age, then inserts a row providing only id.

Execution Table

Step	Column	Value Provided?	DEFAULT Defined?	Value Used	Action
1	id	Yes (1)	No	1	Use provided value
2	name	No	Yes ('Guest')	'Guest'	Use DEFAULT value
3	age	No	Yes (18)	18	Use DEFAULT value
4	Insert row	-	-	(1, 'Guest', 18)	Row inserted with final values

💡 All columns processed; row inserted with provided and default values.

Variable Tracker

Variable	Start	After id	After name	After age	Final
id	NULL	1	1	1	1
name	NULL	NULL	'Guest'	'Guest'	'Guest'
age	NULL	NULL	NULL	18	18

Key Moments - 2 Insights

What happens if no value is provided and no DEFAULT is set?

If no value is given and no DEFAULT exists, SQL inserts NULL or throws an error if the column is NOT NULL. This is shown by the decision in step 3 of the concept flow.

Does DEFAULT value override a provided value?

No, if a value is provided, SQL uses it directly as shown in execution_table step 1 for 'id'. DEFAULT only applies when no value is given.

Visual Quiz - 3 Questions

Look at the execution_table, what value is used for the 'name' column?

A NULL

B 'Guest'

C Empty string

D Provided value

💡 Hint

Check the row where Column is 'name' and Value Provided? is 'No' in execution_table.

At which step is the final row inserted into the table?

A Step 1

B Step 3

C Step 4

D Step 2

💡 Hint

Look for the step with Action 'Row inserted with final values' in execution_table.

If the 'age' column had no DEFAULT and no value was provided, what would happen?

A Use NULL or error if NOT NULL

B Use 0 automatically

C Use previous row's value

D Use 'Guest'

💡 Hint

Refer to the concept_flow decision after 'Is DEFAULT defined?' when answer is No.

Concept Snapshot

DEFAULT values in SQL:
- Used when INSERT omits a column value
- Syntax: column_name TYPE DEFAULT value
- If no value given, DEFAULT applies
- If no DEFAULT, NULL or error if NOT NULL
- Ensures data consistency and reduces insert errors

Full Transcript

When you insert data into a SQL table, each column is checked. If you provide a value, SQL uses it. If you don't, SQL looks for a DEFAULT value for that column. If a DEFAULT exists, SQL uses it automatically. If no DEFAULT exists, SQL inserts NULL or throws an error if the column requires a value. This process ensures your data is complete and consistent without needing to specify every column every time.

Practice

1. What does the DEFAULT keyword do in a SQL table column definition?

easy

A. Deletes the column if no value is provided

B. Prevents any value from being inserted into the column

C. Sets a value automatically if no value is provided during insert

D. Requires the user to always provide a value

Solution

1. Step 1: Understand the purpose of DEFAULT

   The DEFAULT keyword assigns a preset value to a column when no value is given during data insertion.

2. Step 2: Compare options with this purpose

   Only Sets a value automatically if no value is provided during insert correctly describes this automatic assignment behavior.

3. Final Answer:

   Sets a value automatically if no value is provided during insert -> Option C

4. Quick Check:

   DEFAULT = automatic value assignment [OK]

Hint: DEFAULT fills missing values automatically [OK]

Common Mistakes:

• Thinking DEFAULT deletes or restricts data
• Confusing DEFAULT with NOT NULL constraint
• Assuming DEFAULT requires manual input

2. Which of the following is the correct syntax to set a default value of 100 for a column named score in a table?

easy

A. score INT DEFAULT 100

B. score INT SET DEFAULT 100

C. score INT DEFAULT = 100

D. score INT DEFAULT TO 100

Solution

1. Step 1: Recall standard SQL syntax for DEFAULT

   The correct syntax is: column_name data_type DEFAULT value, without extra keywords or symbols.

2. Step 2: Check each option

   score INT DEFAULT 100 matches the correct syntax exactly. Options B, C, and D add incorrect keywords or symbols.

3. Final Answer:

   score INT DEFAULT 100 -> Option A

4. Quick Check:

   DEFAULT syntax = column type DEFAULT value [OK]

Hint: DEFAULT syntax: column type DEFAULT value [OK]

Common Mistakes:

• Adding extra keywords like SET or TO
• Using equals sign (=) after DEFAULT
• Placing DEFAULT before data type

3. Given the table users with column status VARCHAR(10) DEFAULT 'active', what will be the result of this query?

INSERT INTO users (id, name) VALUES (1, 'Alice');
SELECT id, name, status FROM users WHERE id = 1;

medium

A. [1, 'Alice', NULL]

B. [1, 'Alice', 'active']

C. [1, 'Alice', 'inactive']

D. Syntax error

Solution

1. Step 1: Understand DEFAULT behavior on insert

   Since status has a default of 'active' and no value was provided, SQL inserts 'active' automatically.

2. Step 2: Check the SELECT output

   The SELECT query will show id=1, name='Alice', and status='active' as inserted or defaulted.

3. Final Answer:

   [1, 'Alice', 'active'] -> Option B

4. Quick Check:

   DEFAULT fills missing column with 'active' [OK]

Hint: Missing column uses DEFAULT value automatically [OK]

Common Mistakes:

• Assuming missing columns get NULL instead of DEFAULT
• Thinking query causes syntax error
• Confusing default value with another string

4. Consider this table creation statement:

CREATE TABLE orders (
  id INT PRIMARY KEY,
  quantity INT DEFAULT 1,
  price DECIMAL(5,2) DEFAULT 0.00
);

Which of the following INSERT statements will cause an error?

medium

A. INSERT INTO orders (id, price) VALUES (12, 'free');

B. INSERT INTO orders (id, quantity) VALUES (11, DEFAULT);

C. INSERT INTO orders (id) VALUES (10);

D. INSERT INTO orders (id, quantity, price) VALUES (13, 5, 9.99);

Solution

1. Step 1: Analyze each INSERT statement

   INSERT INTO orders (id) VALUES (10); inserts id only; quantity and price get defaults (valid). INSERT INTO orders (id, quantity) VALUES (11, DEFAULT); uses DEFAULT keyword explicitly (valid). INSERT INTO orders (id, price) VALUES (12, 'free'); tries to insert 'free' into price (decimal), causing type error. INSERT INTO orders (id, quantity, price) VALUES (13, 5, 9.99); provides all valid values.

2. Step 2: Identify the error

   Only INSERT INTO orders (id, price) VALUES (12, 'free'); has a data type mismatch causing an error.

3. Final Answer:

   INSERT INTO orders (id, price) VALUES (12, 'free'); -> Option A

4. Quick Check:

   Wrong data type in INSERT causes error [OK]

Hint: Check data types when inserting values [OK]

Common Mistakes:

• Assuming DEFAULT keyword is invalid in VALUES
• Ignoring data type mismatch errors
• Thinking missing columns cause errors

5. You have a table products with columns id INT, name VARCHAR(50), and stock INT DEFAULT 0. You want to insert multiple rows but only specify id and name. Which SQL statement correctly inserts two products using DEFAULT for stock?

hard

A. INSERT INTO products (id, name, stock) VALUES (1, 'Pen', 0), (2, 'Pencil', 0);

B. INSERT INTO products VALUES (1, 'Pen'), (2, 'Pencil');

C. INSERT INTO products (id, name, stock) VALUES (1, 'Pen', DEFAULT), (2, 'Pencil', DEFAULT);

D. INSERT INTO products (id, name) VALUES (1, 'Pen'), (2, 'Pencil');

Solution

1. Step 1: Understand requirement for column specification

   To only specify id and name, list only those columns; omitted columns like stock use their DEFAULT value automatically.

2. Step 2: Evaluate each option

   INSERT INTO products (id, name) VALUES (1, 'Pen'), (2, 'Pencil'); specifies only id and name, stock uses DEFAULT (correct).
   INSERT INTO products VALUES (1, 'Pen'), (2, 'Pencil'); no column list with 2 values (vs 3 columns) causes mismatch error.
   INSERT INTO products (id, name, stock) VALUES (1, 'Pen', DEFAULT), (2, 'Pencil', DEFAULT); specifies stock column (violates 'only id and name').
   INSERT INTO products (id, name, stock) VALUES (1, 'Pen', 0), (2, 'Pencil', 0); specifies stock explicitly with value (not relying on DEFAULT).

3. Final Answer:

   INSERT INTO products (id, name) VALUES (1, 'Pen'), (2, 'Pencil'); -> Option D

4. Quick Check:

   Omit column with DEFAULT = auto-use [OK]

Hint: Omit columns with DEFAULT to use them automatically [OK]

Common Mistakes:

• No column list + fewer values = mismatch error
• Including stock column even with DEFAULT keyword
• Hardcoding default value instead of omitting column