NOW, CURDATE, CURTIME in MySQL - Time & Space Complexity

Time Complexity: NOW, CURDATE, CURTIME

O(n)

Understanding Time Complexity

We want to understand how the time it takes to get the current date and time changes as we ask more times.

How does the cost grow when using NOW, CURDATE, and CURTIME functions repeatedly?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

SELECT NOW();
SELECT CURDATE();
SELECT CURTIME();

-- Using these functions in a query with many rows
SELECT id, NOW() FROM orders;

This code gets the current date and time, date only, or time only. It also shows using NOW() for each row in a table.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Calling NOW(), CURDATE(), or CURTIME() functions.
• How many times: Once if called alone; once per row if used in a query with many rows.

How Execution Grows With Input

Getting the current time once is very fast and does not depend on input size.

Input Size (n)	Approx. Operations
10	10 calls if used per row
100	100 calls if used per row
1000	1000 calls if used per row

Pattern observation: The total work grows linearly with the number of rows when used per row.

Final Time Complexity

Time Complexity: O(n)

This means the time to get the current time grows directly with the number of rows you ask for.

Common Mistake

[X] Wrong: "Calling NOW() once in a query with many rows only runs once and is very cheap."

[OK] Correct: In many SQL engines, NOW() is evaluated once per row, so the cost grows with rows, not constant.

Interview Connect

Understanding how functions like NOW() behave with many rows helps you write efficient queries and shows you think about performance.

Self-Check

"What if we used NOW() in a WHERE clause instead of SELECT? How would the time complexity change?"

Practice

1. What does the MySQL function NOW() return?

easy

A. Only the current date

B. The current date and time

C. Only the current time

D. The current year

Solution

1. Step 1: Understand the function purpose

   NOW() is designed to return both date and time together.

2. Step 2: Compare with other functions

   CURDATE() returns only date, CURTIME() returns only time, so NOW() returns both.

3. Final Answer:

   The current date and time -> Option B

4. Quick Check:

   NOW() = current date + time [OK]

Hint: NOW() gives date and time together [OK]

Common Mistakes:

• Confusing NOW() with CURDATE()
• Thinking NOW() returns only time
• Assuming NOW() returns only year

2. Which of the following is the correct syntax to get only the current date in MySQL?

easy

A. SELECT CURDATE();

B. SELECT NOW();

C. SELECT CURRENT_TIME();

D. SELECT CURTIME();

Solution

1. Step 1: Identify function for current date

   CURDATE() returns only the current date in MySQL.

2. Step 2: Check other options

   NOW() returns date and time, CURRENT_TIME() and CURTIME() return only time.

3. Final Answer:

   SELECT CURDATE(); -> Option A

4. Quick Check:

   CURDATE() = current date only [OK]

Hint: Use CURDATE() for date only [OK]

Common Mistakes:

• Using NOW() when only date is needed
• Confusing CURTIME() with CURDATE()
• Using CURRENT_TIME() for date

3. What will be the output of this query if run at exactly 2024-06-15 14:30:45?

SELECT NOW(), CURDATE(), CURTIME();

medium

A. '2024-06-15 00:00:00', '14:30:45', '2024-06-15'

B. '2024-06-15', '14:30:45', '2024-06-15 14:30:45'

C. '14:30:45', '2024-06-15 14:30:45', '2024-06-15'

D. '2024-06-15 14:30:45', '2024-06-15', '14:30:45'

Solution

1. Step 1: Understand each function output

   NOW() returns full date and time, CURDATE() returns date only, CURTIME() returns time only.

2. Step 2: Match given time values

   At 2024-06-15 14:30:45, NOW() = '2024-06-15 14:30:45', CURDATE() = '2024-06-15', CURTIME() = '14:30:45'.

3. Final Answer:

   '2024-06-15 14:30:45', '2024-06-15', '14:30:45' -> Option D

4. Quick Check:

   NOW() = full datetime, CURDATE() = date, CURTIME() = time [OK]

Hint: NOW() full, CURDATE() date, CURTIME() time [OK]

Common Mistakes:

• Mixing order of outputs
• Assuming CURTIME() returns date
• Confusing NOW() with CURDATE()

4. Identify the error in this query:

SELECT NOW, CURDATE(), CURTIME();

medium

A. NOW is missing parentheses

B. CURDATE() should be CURTIME()

C. CURTIME() is misspelled

D. No error, query is correct

Solution

1. Step 1: Check function syntax

   MySQL functions require parentheses even if no arguments are passed. NOW without () is invalid.

2. Step 2: Verify other functions

   CURDATE() and CURTIME() have correct syntax with parentheses.

3. Final Answer:

   NOW is missing parentheses -> Option A

4. Quick Check:

   Functions need () even if empty [OK]

Hint: Always use () with MySQL functions [OK]

Common Mistakes:

• Omitting parentheses for functions
• Assuming functions can be called without ()
• Thinking CURDATE() is incorrect

5. You want to insert the current date and time into a table's created_at column. Which query correctly uses MySQL functions to do this?

hard

A. INSERT INTO logs (created_at) VALUES (CURDATE());

B. INSERT INTO logs (created_at) VALUES (CURTIME());

C. INSERT INTO logs (created_at) VALUES (NOW());

D. INSERT INTO logs (created_at) VALUES ('CURDATE()');

Solution

1. Step 1: Understand the column data type

   created_at usually stores full date and time, so we need a function that returns both.

2. Step 2: Choose correct function

   NOW() returns current date and time, CURDATE() returns only date, CURTIME() only time, and quoting function names inserts strings, not values.

3. Final Answer:

   INSERT INTO logs (created_at) VALUES (NOW()); -> Option C

4. Quick Check:

   Use NOW() for full datetime insert [OK]

Hint: Use NOW() to insert full date and time [OK]

Common Mistakes:

• Using CURDATE() when time is needed
• Using CURTIME() when date is needed
• Inserting function name as string