DML (INSERT, UPDATE, DELETE) in DBMS Theory - Time & Space Complexity

Time Complexity: DML (INSERT, UPDATE, DELETE)

O(n)

Understanding Time Complexity

When working with database commands like INSERT, UPDATE, and DELETE, it's important to understand how the time to complete these actions changes as the amount of data grows.

We want to know: how does the work needed increase when we add more rows or update more records?

Scenario Under Consideration

Analyze the time complexity of the following SQL commands.

-- Insert multiple rows
INSERT INTO employees (name, age) VALUES ('Alice', 30), ('Bob', 25), ('Carol', 28);

-- Update rows matching a condition
UPDATE employees SET age = age + 1 WHERE department = 'Sales';

-- Delete rows matching a condition
DELETE FROM employees WHERE retired = TRUE;
    

These commands add new data, change existing data, or remove data based on conditions.

Identify Repeating Operations

Look at what repeats when these commands run.

• Primary operation: Scanning rows to find which ones to update or delete.
• How many times: Once for each row in the table or for each row inserted.

How Execution Grows With Input

As the number of rows in the table grows, the time to find and change rows grows too.

Input Size (n)	Approx. Operations
10	About 10 row checks or inserts
100	About 100 row checks or inserts
1000	About 1000 row checks or inserts

Pattern observation: The work grows roughly in direct proportion to the number of rows involved.

Final Time Complexity

Time Complexity: O(n)

This means the time to complete the command grows linearly with the number of rows affected or checked.

Common Mistake

[X] Wrong: "Updating or deleting a few rows always takes the same time no matter how big the table is."

[OK] Correct: Even if only a few rows change, the database often needs to check many rows to find them, so time depends on table size.

Interview Connect

Understanding how data modification commands scale helps you explain database performance clearly and shows you know how data size affects work done.

Self-Check

"What if the table has an index on the column used in the WHERE clause? How would the time complexity change?"

Practice

1. Which DML command is used to add new rows to a database table?

easy

A. INSERT

B. UPDATE

C. DELETE

D. SELECT

Solution

1. Step 1: Understand the purpose of DML commands

   DML commands are used to manipulate data in tables: adding, changing, or removing rows.

2. Step 2: Identify the command for adding rows

   The INSERT command adds new rows, UPDATE changes existing rows, and DELETE removes rows.

3. Final Answer:

   INSERT -> Option A

4. Quick Check:

   Add new rows = INSERT [OK]

Hint: Add new data? Use INSERT command [OK]

Common Mistakes:

• Confusing UPDATE with INSERT
• Using DELETE to add data
• Thinking SELECT adds data

2. Which of the following is the correct syntax to update the salary to 5000 for employee with id 10?

easy

A. UPDATE employees SET salary 5000 WHERE id = 10;

B. UPDATE employees salary = 5000 WHERE id = 10;

C. UPDATE employees SET salary = 5000 WHERE id = 10;

D. UPDATE employees SET salary = 5000;

Solution

1. Step 1: Recall the UPDATE syntax

   The correct syntax is UPDATE table_name SET column = value WHERE condition; to update specific rows.

2. Step 2: Check each option

   UPDATE employees SET salary = 5000 WHERE id = 10; matches the correct syntax with a WHERE clause to target employee id 10. Options B and C miss the SET keyword or equal sign. UPDATE employees SET salary = 5000; lacks WHERE, so it updates all rows.

3. Final Answer:

   UPDATE employees SET salary = 5000 WHERE id = 10; -> Option C

4. Quick Check:

   UPDATE with SET and WHERE = UPDATE employees SET salary = 5000 WHERE id = 10; [OK]

Hint: UPDATE needs SET and WHERE for specific rows [OK]

Common Mistakes:

• Omitting SET keyword
• Missing WHERE clause
• Wrong assignment syntax

3. Consider the table products with columns id, name, and price. What will be the result of this query?

DELETE FROM products WHERE price < 100;

medium

A. All products with price greater than 100 will be removed.

B. All products with price less than 100 will be removed.

C. No products will be removed because condition is incorrect.

D. Syntax error due to missing semicolon.

Solution

1. Step 1: Understand DELETE with WHERE condition

   The DELETE command removes rows matching the WHERE condition. Here, it targets rows where price is less than 100.

2. Step 2: Analyze the condition and syntax

   The condition price < 100 correctly selects products priced below 100. The semicolon is optional in many systems but good practice.

3. Final Answer:

   All products with price less than 100 will be removed. -> Option B

4. Quick Check:

   DELETE with price < 100 removes matching rows [OK]

Hint: DELETE removes rows matching WHERE condition [OK]

Common Mistakes:

• Confusing less than with greater than
• Thinking DELETE needs all rows
• Assuming syntax error without semicolon

4. Identify the error in this SQL statement:

UPDATE employees SET salary = 6000 WHERE;

medium

A. Missing condition after WHERE clause.

B. Incorrect use of SET keyword.

C. Salary value should be in quotes.

D. UPDATE cannot be used without DELETE.

Solution

1. Step 1: Check the UPDATE statement structure

   UPDATE requires SET to assign new values and WHERE to specify which rows to update.

2. Step 2: Identify the error in the WHERE clause

   The statement ends with WHERE but no condition follows, which is a syntax error.

3. Final Answer:

   Missing condition after WHERE clause. -> Option A

4. Quick Check:

   WHERE must have a condition [OK]

Hint: WHERE must always have a condition [OK]

Common Mistakes:

• Leaving WHERE empty
• Putting value in quotes unnecessarily
• Confusing UPDATE with DELETE

5. You want to increase the price by 10% for all products priced below 50. Which SQL statement correctly does this?

hard

A. UPDATE products SET price = price + 0.10 WHERE price < 50;

B. UPDATE products SET price = price + 10 WHERE price < 50;

C. UPDATE products SET price = price * 0.10 WHERE price < 50;

D. UPDATE products SET price = price * 1.10 WHERE price < 50;

Solution

1. Step 1: Understand the requirement to increase price by 10%

   Increasing by 10% means multiplying the current price by 1.10 (100% + 10%).

2. Step 2: Analyze each option

   UPDATE products SET price = price * 1.10 WHERE price < 50; correctly multiplies price by 1.10 for prices less than 50. UPDATE products SET price = price + 10 WHERE price < 50; adds 10 units, not 10%. UPDATE products SET price = price * 0.10 WHERE price < 50; multiplies by 0.10, reducing price. UPDATE products SET price = price + 0.10 WHERE price < 50; adds 0.10 units, a small increase.

3. Final Answer:

   UPDATE products SET price = price * 1.10 WHERE price < 50; -> Option D

4. Quick Check:

   Multiply price by 1.10 to increase by 10% [OK]

Hint: Multiply by 1.10 to increase by 10% [OK]

Common Mistakes:

• Adding fixed amount instead of percentage
• Multiplying by 0.10 reduces price
• Forgetting WHERE clause