LIMIT and OFFSET for pagination in MySQL - Time & Space Complexity

Time Complexity: LIMIT and OFFSET for pagination

O(n)

Understanding Time Complexity

When using LIMIT and OFFSET in MySQL, we want to know how the time to get results changes as we ask for different pages of data.

We ask: How does the work grow when we move to later pages?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

SELECT * FROM products
ORDER BY product_id
LIMIT 10 OFFSET 30;
    

This query gets 10 products starting from the 31st product, ordered by product_id.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Scanning rows to skip OFFSET and then fetching LIMIT rows.
• How many times: The database reads rows up to OFFSET + LIMIT to return the page.

How Execution Grows With Input

As OFFSET grows, the database must skip more rows before returning the page.

Input Size (OFFSET)	Approx. Rows Read
10	About 20 rows (10 skipped + 10 returned)
100	About 110 rows (100 skipped + 10 returned)
1000	About 1010 rows (1000 skipped + 10 returned)

Pattern observation: The work grows roughly in a straight line with OFFSET size.

Final Time Complexity

Time Complexity: O(n)

This means the time to get a page grows linearly with how far into the list you go.

Common Mistake

[X] Wrong: "Using OFFSET doesn't affect performance because we only get a small page of results."

[OK] Correct: The database still reads and skips all rows before the OFFSET, so bigger OFFSET means more work.

Interview Connect

Understanding how LIMIT and OFFSET affect query time helps you explain pagination performance clearly and shows you know how databases handle data retrieval.

Self-Check

"What if we replaced OFFSET with a WHERE condition to start after a known product_id? How would the time complexity change?"

Practice

1. What does the LIMIT clause do in a MySQL query?

easy

A. It controls how many rows the query returns.

B. It controls the order of the rows returned.

C. It filters rows based on a condition.

D. It changes the database schema.

Solution

1. Step 1: Understand the purpose of LIMIT

   The LIMIT clause restricts the number of rows returned by a query.

2. Step 2: Compare with other options

   Ordering is done by ORDER BY, filtering by WHERE, and schema changes by DDL commands, not LIMIT.

3. Final Answer:

   It controls how many rows the query returns. -> Option A

4. Quick Check:

   LIMIT = number of rows [OK]

Hint: LIMIT sets max rows returned, like page size [OK]

Common Mistakes:

• Confusing LIMIT with ORDER BY
• Thinking LIMIT filters rows by condition
• Assuming LIMIT changes table structure

2. Which of the following is the correct syntax using the OFFSET keyword to get 10 rows starting from the 7th row in MySQL?

easy

A. SELECT * FROM table LIMIT 6, 10;

B. SELECT * FROM table LIMIT 10 OFFSET 6;

C. SELECT * FROM table OFFSET 6 LIMIT 10;

D. SELECT * FROM table LIMIT 10, 6;

Solution

1. Step 1: Understand LIMIT and OFFSET syntax

   MySQL supports both LIMIT count OFFSET offset and LIMIT offset, count syntax.

2. Step 2: Identify correct syntax for 10 rows starting at 7th row

   OFFSET 6 means skip first 6 rows, then LIMIT 10 means get next 10 rows. SELECT * FROM table LIMIT 10 OFFSET 6; uses this correctly.

3. Final Answer:

   SELECT * FROM table LIMIT 10 OFFSET 6; -> Option B

4. Quick Check:

   LIMIT count OFFSET offset = correct syntax [OK]

Hint: Use LIMIT count OFFSET offset for clarity [OK]

Common Mistakes:

• Swapping LIMIT and OFFSET values
• Using OFFSET before LIMIT incorrectly
• Confusing LIMIT offset,count with LIMIT count OFFSET offset

3. Given the table users with rows numbered by id from 1 to 20, what rows will this query return?
SELECT * FROM users ORDER BY id ASC LIMIT 5 OFFSET 10;

medium

A. Rows with id 1 to 5

B. Rows with id 6 to 10

C. Rows with id 11 to 15

D. Rows with id 16 to 20

Solution

1. Step 1: Understand OFFSET and LIMIT effect on rows

   OFFSET 10 skips first 10 rows (id 1 to 10), LIMIT 5 returns next 5 rows.

2. Step 2: Identify which rows are returned

   After skipping 10 rows, rows with id 11 to 15 are returned.

3. Final Answer:

   Rows with id 11 to 15 -> Option C

4. Quick Check:

   OFFSET 10 + LIMIT 5 = rows 11-15 [OK]

Hint: OFFSET skips rows, LIMIT picks next rows [OK]

Common Mistakes:

• Counting OFFSET as inclusive row
• Mixing up OFFSET and LIMIT order
• Assuming OFFSET starts at 1 instead of 0

4. Identify the error in this query intended to get 5 rows starting from the 3rd row:
SELECT * FROM products LIMIT OFFSET 3, 5;

medium

A. Syntax error: LIMIT should come before OFFSET without comma.

B. OFFSET cannot be used with LIMIT in this way.

C. No error; query is correct.

D. LIMIT and OFFSET are reversed in order.

Solution

1. Step 1: Check correct syntax for LIMIT and OFFSET

   Correct syntax is either LIMIT count OFFSET offset or LIMIT offset, count without the word OFFSET in the second form.

2. Step 2: Identify error in given query

   The query uses both LIMIT and OFFSET keywords together incorrectly with a comma, causing syntax error.

3. Final Answer:

   Syntax error: LIMIT should come before OFFSET without comma. -> Option A

4. Quick Check:

   LIMIT offset,count or LIMIT count OFFSET offset [OK]

Hint: Use either LIMIT offset,count or LIMIT count OFFSET offset [OK]

Common Mistakes:

• Mixing LIMIT and OFFSET keywords with commas
• Using OFFSET before LIMIT
• Adding OFFSET keyword with comma syntax

5. You want to display page 4 of a product list, showing 15 products per page, ordered by created_at descending. Which query correctly uses LIMIT and OFFSET?

hard

A. SELECT * FROM products ORDER BY created_at DESC LIMIT 45 OFFSET 15;

B. SELECT * FROM products ORDER BY created_at DESC LIMIT 60 OFFSET 15;

C. SELECT * FROM products ORDER BY created_at DESC LIMIT 15, 45;

D. SELECT * FROM products ORDER BY created_at DESC LIMIT 15 OFFSET 45;

Solution

1. Step 1: Calculate OFFSET for page 4 with 15 items per page

   Page 1 shows rows 0-14, page 2 rows 15-29, page 3 rows 30-44, so page 4 starts at offset 45.

2. Step 2: Write query with LIMIT and OFFSET

   LIMIT 15 means 15 rows per page, OFFSET 45 skips first 45 rows. SELECT * FROM products ORDER BY created_at DESC LIMIT 15 OFFSET 45; matches this.

3. Final Answer:

   SELECT * FROM products ORDER BY created_at DESC LIMIT 15 OFFSET 45; -> Option D

4. Quick Check:

   OFFSET = (page-1)*page_size = 45 [OK]

Hint: OFFSET = (page-1) * page_size, LIMIT = page_size [OK]

Common Mistakes:

• Swapping LIMIT and OFFSET values
• Calculating OFFSET incorrectly
• Using LIMIT as total rows instead of per page