✅Top 10 SQL Interview Questions 💼📊 

1️⃣ What is a CTE (Common Table Expression)? 
A temporary result set used for complex queries.

WITH temp AS (SELECT * FROM employees WHERE salary > 50000)  
SELECT * FROM temp;

2️⃣ Difference between RANK(), DENSE_RANK(), and ROW_NUMBER()?
⦁ ROW_NUMBER(): Unique sequence number
⦁ RANK(): Skips ranks for ties
⦁ DENSE_RANK(): No gaps in ranking

3️⃣ What is a window function? 
Calculations across a set of rows related to the current row, using OVER(PARTITION BY…).

4️⃣ Difference between WHERE and HAVING?
⦁ WHERE: Filters rows before aggregation
⦁ HAVING: Filters rows after aggregation

5️⃣ What is indexing and how does it work? 
Speeds up queries by creating fast-lookups, like a book’s index.

6️⃣ How to remove duplicate records in SQL?

SELECT DISTINCT * FROM table_name;

7️⃣ Difference between UNION and UNION ALL?
⦁ UNION: Removes duplicates
⦁ UNION ALL: Includes all rows

8️⃣ How to update data using JOIN?

UPDATE emp  
SET emp.salary = dept.bonus  
FROM emp  
JOIN dept ON emp.dept_id = dept.id;

9️⃣ Explain COALESCE() vs ISNULL().
⦁ COALESCE(): Returns first non-null from a list
⦁ ISNULL(): Replaces NULL with a specified value (SQL Server)

🔟 What is a correlated subquery? 
A subquery referencing outer query columns, executed row-by-row.

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