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

Time Complexity: ROUND, CEIL, FLOOR

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run ROUND, CEIL, and FLOOR functions changes as the amount of data grows.

How does the execution time grow when these functions are applied to many rows?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

SELECT 
  ROUND(price, 2) AS rounded_price,
  CEIL(quantity) AS ceil_quantity,
  FLOOR(discount) AS floor_discount
FROM sales_data;
    

This query applies ROUND, CEIL, and FLOOR functions to columns in each row of the sales_data table.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Applying each function (ROUND, CEIL, FLOOR) once per row.
• How many times: Once for every row in the sales_data table.

How Execution Grows With Input

Each row requires a fixed number of function calls, so the total work grows as the number of rows grows.

Input Size (n)	Approx. Operations
10	30 (3 functions x 10 rows)
100	300 (3 functions x 100 rows)
1000	3000 (3 functions x 1000 rows)

Pattern observation: The total operations increase directly with the number of rows.

Final Time Complexity

Time Complexity: O(n)

This means the time to run these functions grows in a straight line as the number of rows increases.

Common Mistake

[X] Wrong: "These functions take longer on bigger numbers or decimals, so time grows faster than the number of rows."

[OK] Correct: Each function runs in constant time per row regardless of the number size; only the number of rows affects total 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 nested subquery that also uses ROUND on each row? How would the time complexity change?"

Practice

1. What does the SQL function CEIL(4.3) return?

easy

A. 5

B. 4

C. 4.3

D. 3

Solution

1. Step 1: Understand CEIL function behavior

   The CEIL function rounds a number up to the nearest integer.

2. Step 2: Apply CEIL to 4.3

   Since 4.3 is between 4 and 5, CEIL rounds it up to 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   CEIL(4.3) = 5 [OK]

Hint: CEIL always rounds up to next integer [OK]

Common Mistakes:

• Confusing CEIL with FLOOR
• Thinking CEIL rounds to nearest integer
• Assuming CEIL returns decimal part

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

easy

A. ROUND(123.456.2)

B. ROUND(123.456, 2)

C. ROUND(123.456 TO 2)

D. ROUND(123.456; 2)

Solution

1. Step 1: Recall ROUND function syntax

   ROUND(number, decimal_places) is the correct syntax in SQL.

2. Step 2: Check each option

   Only ROUND(123.456, 2) uses ROUND with a comma separating number and decimal places.

3. Final Answer:

   ROUND(123.456, 2) -> Option B

4. Quick Check:

   ROUND(number, decimals) syntax [OK]

Hint: Use comma between number and decimals in ROUND [OK]

Common Mistakes:

• Using dot instead of comma
• Using semicolon instead of comma
• Adding extra keywords like TO

3. What is the result of the SQL query:
SELECT FLOOR(7.9), CEIL(7.1), ROUND(7.5) FROM dual;

medium

A. (7, 8, 7)

B. (8, 7, 7)

C. (7, 7, 8)

D. (7, 8, 8)

Solution

1. Step 1: Evaluate FLOOR(7.9)

   FLOOR rounds down to 7.

2. Step 2: Evaluate CEIL(7.1)

   CEIL rounds up to 8.

3. Step 3: Evaluate ROUND(7.5)

   ROUND rounds to nearest integer; 7.5 rounds up to 8.

4. Final Answer:

   (7, 8, 8) -> Option D

5. Quick Check:

   FLOOR=7, CEIL=8, ROUND=8 [OK]

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

Common Mistakes:

• Mixing FLOOR and CEIL results
• Assuming ROUND always rounds down
• Confusing ROUND behavior at .5

4. Identify the error in this SQL query:
SELECT CEIL(5.7 2) FROM dual;

medium

A. CEIL does not accept decimal numbers

B. CEIL requires two arguments

C. Missing comma between arguments

D. Syntax is correct

Solution

1. Step 1: Check CEIL function syntax

   CEIL takes only one argument, a number, with no second argument.

2. Step 2: Analyze given query

   The query has '5.7 2' without a comma, which is invalid syntax.

3. Final Answer:

   Missing comma between arguments -> Option C

4. Quick Check:

   CEIL needs one number, no extra arguments [OK]

Hint: Check commas between function arguments [OK]

Common Mistakes:

• Adding extra arguments to CEIL
• Forgetting commas between arguments
• Assuming CEIL rounds decimals differently

5. You have a table prices with a column cost containing decimal values. You want to create a new column cost_rounded that rounds cost to the nearest integer but always rounds .5 up. Which SQL expression should you use?

hard

A. FLOOR(cost + 0.5)

B. CEIL(cost - 0.5)

C. ROUND(cost)

D. CEIL(cost)

Solution

1. Step 1: Understand rounding .5 up requirement

   Standard ROUND rounds .5 up, but behavior can vary by SQL dialect.

2. Step 2: Analyze options

   FLOOR(cost + 0.5) uses FLOOR(cost + 0.5), which shifts values so .5 rounds up correctly.

3. Step 3: Verify other options

   ROUND(cost) may round .5 up but depends on implementation; B and D do not correctly round .5 up.

4. Final Answer:

   FLOOR(cost + 0.5) -> Option A

5. Quick Check:

   Use FLOOR(x + 0.5) to round .5 up reliably [OK]

Hint: Add 0.5 then FLOOR to round .5 up [OK]

Common Mistakes:

• Assuming ROUND always rounds .5 up
• Using CEIL directly without adjustment
• Using FLOOR without adding 0.5