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SQL REVISION SERIES QUESTION 9/100

Q9. What is the difference between WHERE and HAVING clause?

WHERE and HAVING are both used to filter data, but they work at different stages.

WHERE filters rows before GROUP BY.

HAVING filters groups after GROUP BY.

WHERE cannot use aggregate functions like SUM(), COUNT(), AVG().

HAVING can use aggregate functions.

Interview Twist:

• HAVING can be used without GROUP BY.
• Aggregate functions cannot be used in WHERE.
• WHERE executes before HAVING.

Easy Interview Line:

WHERE filters rows before grouping.

HAVING filters groups after grouping.

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SQL REVISION SERIES QUESTION 7/100

Q7. What is the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL JOIN?

JOINS are used to combine data from multiple tables using a related column.

INNER JOIN returns only matching rows from both tables.

LEFT JOIN returns all rows from the left table and matching rows from the right table.

RIGHT JOIN returns all rows from the right table and matching rows from the left table.

FULL JOIN returns all rows from both tables, including unmatched rows.

This is one of the most important SQL interview questions for beginners and data analysts.

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1 week ago | [YT] | 3

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1 week ago | [YT] | 3

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SQL REVISION SERIES – QUESTION 6/100

Q6. What is the difference between WHERE and HAVING?

WHERE and HAVING are both used to filter data, but they work at different stages of query execution.

• WHERE filters rows before GROUP BY
• HAVING filters grouped data after GROUP BY

Quick Revision:

• WHERE = Row-level filtering
• HAVING = Group-level filtering
• WHERE cannot use aggregate functions directly
• HAVING is used with aggregate functions like SUM(), COUNT(), AVG()

Example:
SELECT Dept, SUM(Salary)
FROM Employees
GROUP BY Dept
HAVING SUM(Salary) > 120000;

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1 week ago | [YT] | 4

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Behind every graph is a story of consistency.
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1 week ago | [YT] | 3

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SQL REVISION SERIES – QUESTION 5/100

Q5. What is the difference between DELETE, TRUNCATE and DROP?

These three SQL commands are used to remove data, but they work differently.

• DELETE → Removes selected rows using WHERE clause
• TRUNCATE → Removes all rows quickly, table structure remains
• DROP → Deletes the complete table including structure

Quick Revision:
• DELETE = Row by row deletion
• TRUNCATE = Fast full table cleanup
• DROP = Complete table removal

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1 week ago | [YT] | 5

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🔥 SQL REVISION SERIES – QUESTION 4/100 🔥

📌 Q4. What is Normalization in SQL and why is it important?

✅ Reduces data redundancy
✅ Improves data consistency
✅ Prevents update, insert & delete anomalies
✅ Makes database easy to maintain

📚 Common Normal Forms:
• 1NF
• 2NF
• 3NF
• BCNF

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