Questions

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1What are functions and operators in MySQL, and how are they different?

2What is the difference between single-row functions and aggregate functions?

3Give examples of commonly used string functions in MySQL.

4What is the use of the CONCAT() function? How is it different from using ||?

5Explain the difference between NOW(), CURDATE(), and SYSDATE().

6What are arithmetic operators in MySQL? Give examples.

7What is the difference between the = and <=> operators?

8What does the DISTINCT keyword do when used with aggregate functions like COUNT()?

9What is the difference between IFNULL() and COALESCE() functions?

10What are logical operators in MySQL, and how do AND, OR, and NOT work?

11What is the difference between LENGTH() and CHAR_LENGTH() functions?

12How does MySQL handle type conversion when using operators on different data types?

13What is the difference between ROUND(), TRUNCATE(), and FORMAT() functions?

14Explain how CASE and IF() functions can be used for conditional logic.

15What are comparison operators in MySQL, and how are they used with NULL values?

16How do aggregate functions like AVG(), SUM(), and MAX() behave when NULLs are present?

17Explain the use of REGEXP and LIKE operators. What’s the difference between them?

18What are user-defined functions (UDFs), and how do they differ from stored procedures?

19What are the differences between DATE_ADD() and ADDDATE() functions?

20How can you use STR_TO_DATE() and DATE_FORMAT() for converting and displaying date values?

21How does MySQL internally optimize and cache results of deterministic functions?

22What are window functions in MySQL 8.0, and how do they differ from aggregate functions?

23Explain the difference between RANK(), DENSE_RANK(), and ROW_NUMBER() window functions.

24How does MySQL evaluate operator precedence when multiple operators are used in a single expression?

25Can functions be used in the WHERE clause? What are the performance implications?

26How can you use JSON_EXTRACT() and JSON_CONTAINS() to work with JSON data in MySQL?

27What are the performance trade-offs of using scalar functions inside JOIN or GROUP BY clauses?

28Explain how collation affects comparison operators in string functions.

29What are deterministic and non-deterministic functions? How does this affect replication and indexes?

30How would you combine multiple functions and operators to clean, transform, and aggregate data efficiently in one query?

mysql

Functions and Operators

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Explain the difference between NOW(), CURDATE(), and SYSDATE().

Understanding NOW(), CURDATE(), and SYSDATE() in MySQL

MySQL provides several date and time functions, and three commonly used ones are NOW(), CURDATE(), and SYSDATE(). Although they look similar, they return different values and behave differently in queries.

1. NOW() — Current Date and Time (Fixed During the Query)

1
Returns the current date and time based on the server's time zone.
2
NOW() is evaluated once per statement.
3
All rows in a long-running query see the same timestamp.

Example

2. CURDATE() — Only the Current Date

1
Returns only the current date (YYYY-MM-DD).
2
No time component.
3
Useful for filtering rows by the current day.

Example

3. SYSDATE() — Current Date and Time (Evaluated Immediately)

1
Returns the current date and time, like NOW().
2
BUT unlike NOW(), SYSDATE() is evaluated in real time.
3
Each row can see different timestamps in long-running queries.
4
This can lead to inconsistent results in replication unless sysdate-is-now is enabled.

Example

4. Key Differences at a Glance

1
NOW() → Returns date & time; fixed per query.
2
CURDATE() → Returns only date; no time part.
3
SYSDATE() → Returns date & time; evaluated at execution time (can change row-to-row).

In summary: Use NOW() when you want a consistent timestamp across the query, CURDATE() for date-only operations, and SYSDATE() when you need the exact execution moment for each row.