ROUND, CEIL, FLOOR in MySQL - Time & Space Complexity

Time Complexity: ROUND, CEIL, FLOOR

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run rounding functions changes as the amount of data grows.

How does using ROUND, CEIL, or FLOOR on many numbers affect performance?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

SELECT ROUND(price, 2) AS rounded_price,
       CEIL(price) AS ceiling_price,
       FLOOR(price) AS floor_price
FROM products;
    

This query applies rounding functions to each price in the products table.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Applying ROUND, CEIL, and FLOOR to each row's price.
• How many times: Once per row in the products table.

How Execution Grows With Input

Each row requires the functions to run once, so the total work grows as the number of rows grows.

Input Size (n)	Approx. Operations
10	About 30 function calls
100	About 300 function calls
1000	About 3000 function calls

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

Final Time Complexity

Time Complexity: O(n)

This means the time to run grows in a straight line with the number of rows processed.

Common Mistake

[X] Wrong: "Rounding functions run instantly no matter how many rows there are."

[OK] Correct: Each row needs its own calculation, so more rows mean more work and more time.

Interview Connect

Understanding how simple functions scale with data size helps you explain query performance clearly and confidently.

Self-Check

"What if we added a WHERE clause to filter rows before applying ROUND, CEIL, and FLOOR? How would the time complexity change?"

Practice

1. Which MySQL function rounds a number to the nearest whole number or specified decimal place?

easy

A. CEIL

B. ROUND

C. FLOOR

D. TRUNCATE

Solution

1. Step 1: Understand the function purpose

   ROUND changes a number to the nearest integer or decimal place, unlike CEIL or FLOOR which only round up or down.

2. Step 2: Compare with other functions

   CEIL always rounds up, FLOOR always rounds down, TRUNCATE cuts decimals without rounding.

3. Final Answer:

   ROUND -> Option B

4. Quick Check:

   ROUND = nearest rounding [OK]

Hint: ROUND = nearest number, CEIL up, FLOOR down [OK]

Common Mistakes:

• Confusing CEIL with ROUND
• Thinking FLOOR rounds to nearest
• Assuming TRUNCATE rounds

2. Which of the following is the correct syntax to round the number 3.14159 to 2 decimal places in MySQL?

easy

A. ROUND(3.14159, 2)

B. CEIL(3.14159, 2)

C. FLOOR(3.14159, 2)

D. ROUND(3.14159. 2)

Solution

1. Step 1: Check function usage

   ROUND accepts two arguments: the number and the decimal places to round to.

2. Step 2: Identify correct syntax

   ROUND(3.14159, 2) is correct; CEIL and FLOOR do not take decimal places, and ROUND(3.14159. 2) has a syntax error with a dot instead of a comma.

3. Final Answer:

   ROUND(3.14159, 2) -> Option A

4. Quick Check:

   ROUND(number, decimals) syntax [OK]

Hint: ROUND(number, decimals) uses comma, CEIL/FLOOR no decimals [OK]

Common Mistakes:

• Using CEIL or FLOOR with decimal places
• Using dot instead of comma between arguments
• Missing second argument for ROUND

3. What is the result of the query: SELECT CEIL(4.2), FLOOR(4.8), ROUND(4.5);?

medium

A. 5, 4, 5

B. 4, 4, 4

C. 5, 5, 5

D. 4, 5, 4

Solution

1. Step 1: Evaluate CEIL(4.2)

   CEIL rounds up to the next whole number, so CEIL(4.2) = 5.

2. Step 2: Evaluate FLOOR(4.8) and ROUND(4.5)

   FLOOR rounds down, so FLOOR(4.8) = 4. ROUND rounds to nearest integer, 4.5 rounds up to 5.

3. Final Answer:

   5, 4, 5 -> Option A

4. Quick Check:

   CEIL=5, FLOOR=4, ROUND=5 [OK]

Hint: CEIL up, FLOOR down, ROUND nearest (0.5 rounds up) [OK]

Common Mistakes:

• Thinking FLOOR(4.8) is 5
• Rounding 4.5 down instead of up
• Confusing CEIL and FLOOR

4. Identify the error in this query: SELECT CEIL(5.7, 1), FLOOR(3.2), ROUND(2.718);

medium

A. ROUND requires two arguments

B. FLOOR requires two arguments

C. CEIL does not accept two arguments

D. No error, query is correct

Solution

1. Step 1: Check CEIL function arguments

   CEIL only accepts one argument (the number). Passing two arguments causes an error.

2. Step 2: Verify FLOOR and ROUND usage

   FLOOR accepts one argument, ROUND can accept one or two arguments. Both FLOOR(3.2) and ROUND(2.718) are valid.

3. Final Answer:

   CEIL does not accept two arguments -> Option C

4. Quick Check:

   CEIL = 1 argument only [OK]

Hint: CEIL/FLOOR take 1 argument; ROUND can take 1 or 2 [OK]

Common Mistakes:

• Passing two arguments to CEIL or FLOOR
• Assuming ROUND always needs two arguments
• Thinking query has no errors

5. You have a table prices with a column cost containing decimal values. You want to create a new column rounded_cost that rounds cost to the nearest integer but always rounds up if the decimal part is exactly 0.5. Which MySQL expression achieves this?

hard

A. ROUND(cost)

B. IF(cost - FLOOR(cost) = 0.5, FLOOR(cost), ROUND(cost))

C. CEIL(cost - 0.5)

D. CEIL(cost)

Solution

1. Step 1: Understand rounding rules

   In MySQL, ROUND rounds to nearest integer; halfway cases (.5) round away from zero (up for positive numbers).

2. Step 2: Select the correct expression

   CEIL(cost) always rounds up, so it rounds up when decimal part is exactly 0.5, fulfilling the requirement.

3. Step 3: Analyze other options

   A rounds .5 up normally, but does not always round up if decimal is exactly 0.5 (it rounds to nearest). B rounds down for exactly .5, which is incorrect. D fails on .5 cases as CEIL(cost - 0.5) can round down.

4. Final Answer:

   CEIL(cost) -> Option D

5. Quick Check:

   CEIL(cost) always rounds up [OK]

Hint: CEIL(cost) always rounds up, including .5 cases [OK]

Common Mistakes:

• Assuming ROUND always rounds .5 up as needed
• Using conditional that rounds .5 down
• CEIL(cost - 0.5) fails on .5 cases