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.