✅ SQL Functions Interview Questions with Answers 🎯📚

1️⃣ Q: What is the difference between COUNT(*) and COUNT(column_name)?
A:
- COUNT(*) counts all rows, including those with NULLs.
- COUNT(column_name) counts only rows where the column is NOT NULL.

2️⃣ Q: When would you use GROUP BY with aggregate functions?
A:
Use GROUP BY when you want to apply aggregate functions per group (e.g., department-wise total salary):

SELECT department, SUM(salary) FROM employees GROUP BY department;

3️⃣ Q: What does the COALESCE() function do?
A:
COALESCE() returns the first non-null value from the list of arguments.
Example:

SELECT COALESCE(phone, 'N/A') FROM users;

4️⃣ Q: How does the CASE statement work in SQL?
A:
CASE is used for conditional logic inside queries.
Example:

SELECT name,  
  CASE  
    WHEN score >= 90 THEN 'A'  
    WHEN score >= 75 THEN 'B'  
    ELSE 'C'  
  END AS grade  
FROM students;

5️⃣ Q: What’s the use of SUBSTRING() function?
A:
It extracts a part of a string.
Example:

SELECT SUBSTRING('DataScience', 1, 4); -- Output: Data

6️⃣ Q: What’s the output of LENGTH('SQL')?
A:
It returns the length of the string: 3

7️⃣ Q: How do you find the number of days between two dates?
A:
Use DATEDIFF(end_date, start_date)
Example:

SELECT DATEDIFF('2026-01-10', '2026-01-05'); -- Output: 5

8️⃣ Q: What does ROUND() do in SQL?
A:
It rounds a number to the specified decimal places.
Example:

SELECT ROUND(3.456, 2); -- Output: 3.46

💡 Pro Tip: Always mention real use cases when answering — it shows practical understanding.

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