INSERT with DEFAULT values in SQL - Time & Space Complexity

Time Complexity: INSERT with DEFAULT values

O(n)

Understanding Time Complexity

When we add new rows to a database table using default values, it is important to understand how the time it takes grows as we add more rows.

We want to know how the work done changes when inserting many rows with default values.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

INSERT INTO employees DEFAULT VALUES;

INSERT INTO employees DEFAULT VALUES;

-- Repeat many times to add multiple rows
    

This code inserts one new row into the employees table using all default values each time it runs.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Inserting a single row with default values.
• How many times: Once per INSERT statement executed.

How Execution Grows With Input

Each time we run the INSERT, the database does a fixed amount of work to add one row.

Input Size (n)	Approx. Operations
10	10 insert operations
100	100 insert operations
1000	1000 insert operations

Pattern observation: The work grows directly with the number of rows inserted.

Final Time Complexity

Time Complexity: O(n)

This means the time to insert rows grows in a straight line as you add more rows one by one.

Common Mistake

[X] Wrong: "Inserting rows with default values takes the same time no matter how many rows I add."

[OK] Correct: Each row requires a separate insert operation, so more rows mean more work and more time.

Interview Connect

Understanding how inserting rows scales helps you explain database performance clearly and shows you know how operations grow with data size.

Self-Check

"What if we insert multiple rows in a single INSERT statement instead of one at a time? How would the time complexity change?"

Practice

1. What does the SQL statement INSERT INTO table_name DEFAULT VALUES; do?

easy

A. Inserts a new row using all default values for columns

B. Deletes all rows from the table

C. Updates existing rows with default values

D. Selects all rows with default values

Solution

1. Step 1: Understand the INSERT statement

   The statement inserts a new row into the table.

2. Step 2: Recognize DEFAULT VALUES usage

   Using DEFAULT VALUES means the new row uses default values defined for each column.

3. Final Answer:

   Inserts a new row using all default values for columns -> Option A

4. Quick Check:

   INSERT ... DEFAULT VALUES inserts default row [OK]

Hint: DEFAULT VALUES inserts a row with all default column values [OK]

Common Mistakes:

• Thinking it deletes or updates rows
• Confusing DEFAULT VALUES with SELECT
• Assuming it inserts NULLs instead of defaults

2. Which of the following is the correct syntax to insert a row with default values in SQL?

easy

A. INSERT INTO table_name VALUES (DEFAULT);

B. INSERT INTO table_name VALUES DEFAULT;

C. INSERT DEFAULT INTO table_name VALUES;

D. INSERT INTO table_name DEFAULT VALUES;

Solution

1. Step 1: Recall correct syntax for default insertion

   The correct syntax is INSERT INTO table_name DEFAULT VALUES; to insert a row with all default values.

2. Step 2: Check each option

   INSERT INTO table_name DEFAULT VALUES; matches the correct syntax exactly; others are invalid or incorrect.

3. Final Answer:

   INSERT INTO table_name DEFAULT VALUES; -> Option D

4. Quick Check:

   Correct syntax = INSERT INTO ... DEFAULT VALUES [OK]

Hint: Remember: DEFAULT VALUES comes after table name in INSERT [OK]

Common Mistakes:

• Placing DEFAULT inside VALUES parentheses
• Swapping DEFAULT and VALUES keywords
• Using incorrect keyword order

3. Given the table users(id INT DEFAULT 0, name VARCHAR(20) DEFAULT 'guest'), what will be the result of INSERT INTO users DEFAULT VALUES; SELECT * FROM users; after running on an empty table?

medium

A. Empty result set

B. [{"id": 0, "name": "guest"}]

C. Syntax error

D. [{"id": NULL, "name": NULL}]

Solution

1. Step 1: Understand table defaults

   The table has default values: id = 0, name = 'guest'.

2. Step 2: Effect of INSERT DEFAULT VALUES

   Inserting default values adds a row with id=0 and name='guest'.

3. Step 3: SELECT result

   After insertion, selecting all rows returns one row with those default values.

4. Final Answer:

   [{"id": 0, "name": "guest"}] -> Option B

5. Quick Check:

   Defaults inserted row = id 0, name guest [OK]

Hint: Defaults fill columns if no values given [OK]

Common Mistakes:

• Expecting NULL instead of default values
• Assuming syntax error due to DEFAULT VALUES
• Thinking table remains empty

4. Consider the table products(id INT NOT NULL, name VARCHAR(20) DEFAULT 'unknown'). What error occurs when running INSERT INTO products DEFAULT VALUES;?

medium

A. Error: Column 'id' has no default and no value provided

B. No error, inserts row with NULL id and 'unknown' name

C. Syntax error due to DEFAULT VALUES usage

D. Inserts row with id=0 and name='unknown'

Solution

1. Step 1: Check columns for defaults

   The 'id' column has no default and is NOT nullable.

2. Step 2: Effect of INSERT DEFAULT VALUES

   Trying to insert default values fails because 'id' has no default and no value is given.

3. Final Answer:

   Error: Column 'id' has no default and no value provided -> Option A

4. Quick Check:

   Missing default for NOT NULL column causes error [OK]

Hint: All columns need defaults or allow NULL for DEFAULT VALUES [OK]

Common Mistakes:

• Assuming NULL is inserted automatically
• Thinking DEFAULT VALUES always works
• Confusing syntax error with constraint error

5. You have a table orders(order_id INT PRIMARY KEY DEFAULT nextval('order_seq'), status VARCHAR(10) DEFAULT 'pending', created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP). Which SQL statement correctly inserts a new order using all defaults except setting status to 'shipped'?

hard

A. INSERT INTO orders (order_id, status, created_at) VALUES (DEFAULT, DEFAULT, 'shipped');

B. INSERT INTO orders DEFAULT VALUES SET status = 'shipped';

C. INSERT INTO orders (status) VALUES ('shipped');

D. INSERT INTO orders (DEFAULT, 'shipped', DEFAULT);

Solution

1. Step 1: Understand default and explicit values

   We want to use defaults for order_id and created_at, but set status explicitly.

2. Step 2: Check each option

   INSERT INTO orders (status) VALUES ('shipped'); correctly lets the DBMS use defaults implicitly for unspecified columns order_id and created_at. INSERT INTO orders DEFAULT VALUES SET status = 'shipped'; is invalid syntax. INSERT INTO orders (order_id, status, created_at) VALUES (DEFAULT, DEFAULT, 'shipped'); incorrectly sets status to its default ('pending') and attempts to set created_at to 'shipped' causing a type mismatch error. INSERT INTO orders (DEFAULT, 'shipped', DEFAULT); is invalid syntax (missing VALUES keyword).

3. Final Answer:

   INSERT INTO orders (status) VALUES ('shipped'); -> Option C

4. Quick Check:

   Unspecified columns use their default values automatically [OK]

Hint: Unspecified columns in INSERT use column defaults [OK]

Common Mistakes:

• Thinking omitting columns always causes errors
• Using DEFAULT VALUES with SET syntax
• Misplacing values and DEFAULT keywords in VALUES clause