SQL Interview ke Liye Sabse Important Questions

Agar aap SQL interview ki taiyari kar rahe ho, to yeh article aapke liye perfect hai! Yahan humne basic, intermediate, advanced, practical aur behavioral questions ko Hinglish mein cover kiya hai, jo aksar interviews mein puche jaate hain. Chalo shuru karte hain!





✅ Basic SQL Interview Questions

  1. SQL Kya Hai?
    SQL (Structured Query Language) ek programming language hai jo databases se data ko manage aur manipulate karne ke liye use hoti hai. Iska use queries likhne, data retrieve karne, aur database structure ko modify karne ke liye hota hai.

  2. SQL Statements ke Different Types Kya Hain?
    SQL mein 5 main types ke statements hote hain:

    • DDL (Data Definition Language): Table aur schema create, alter, drop karne ke liye (e.g., CREATE, ALTER, DROP).

    • DML (Data Manipulation Language): Data insert, update, delete karne ke liye (e.g., INSERT, UPDATE, DELETE).

    • DCL (Data Control Language): Permissions aur access control ke liye (e.g., GRANT, REVOKE).

    • TCL (Transaction Control Language): Transactions manage karne ke liye (e.g., COMMIT, ROLLBACK).

    • DQL (Data Query Language): Data retrieve karne ke liye (e.g., SELECT).

  3. DELETE, TRUNCATE, aur DROP Mein Kya Fark Hai?

    • DELETE: Specific rows ko table se remove karta hai (WHERE clause ke saath). Data delete hota hai, par table structure baki rehta hai.

    • TRUNCATE: Table ke saare records ek baar mein delete karta hai, par structure rehta hai. Faster hai DELETE se.

    • DROP: Pura table hi drop ho jata hai, structure bhi chala jata hai.

  4. Different Types ke JOINs Kya Hote Hain?

    • INNER JOIN: Sirf matching records dono tables se return karta hai.

    • LEFT JOIN: Left table ke saare records aur right table ke matching records.

    • RIGHT JOIN: Right table ke saare records aur left table ke matching records.

    • FULL JOIN: Dono tables ke saare records, matching ya non-matching.

    • SELF JOIN: Ek table ko khud se join karna.

  5. WHERE aur HAVING Mein Kya Difference Hai?

    • WHERE: Rows ko filter karta hai before grouping (SELECT ke saath use hota hai).

    • HAVING: Groups ko filter karta hai after GROUP BY clause.

  6. Primary Key Kya Hai?
    Primary Key ek unique identifier hai jo table ke har row ko uniquely identify karta hai. Yeh NULL nahi ho sakta.

  7. Foreign Key Kya Hai?
    Foreign Key ek column ya columns ka set hai jo ek table ko doosre table ki Primary Key se connect karta hai, referential integrity maintain karne ke liye.

  8. UNION aur UNION ALL Mein Kya Difference Hai?

    • UNION: Duplicate rows remove karke combined result deta hai.

    • UNION ALL: Duplicates ke saath saara data combine karta hai (faster hai).

  9. GROUP BY Ka Use Kya Hai?
    GROUP BY data ko groups mein organize karta hai (e.g., sum, count, avg calculate karne ke liye).

  10. NULL Value Kya Hai? Isse Kaise Handle Karte Ho?
    NULL means missing ya unknown value. Isko handle karne ke liye:

    • IS NULL / IS NOT NULL use karo queries mein.

    • COALESCE() ya IFNULL() se default value set karo.

✅ Intermediate SQL Interview Questions

  1. Normalization Kya Hai? 1NF, 2NF, 3NF Explain Karo.
    Normalization database ko redundant data aur anomalies se bachane ke liye structure ko optimize karta hai:

    • 1NF (First Normal Form): Har column atomic hona chahiye, no repeating groups.

    • 2NF: 1NF ke saath, non-key attributes fully dependent on Primary Key hone chahiye.

    • 3NF: 2NF ke saath, no transitive dependency (non-key attributes depend only on Primary Key).

  2. Denormalization Kya Hai?
    Denormalization performance improve karne ke liye redundant data add karta hai, especially read-heavy applications mein.

  3. Subquery Kya Hai? Correlated Subquery Kya Hoti Hai?

    • Subquery: Ek query ke andar doosri query hoti hai.

    • Correlated Subquery: Inner query outer query ke rows pe depend karti hai, har row ke liye execute hoti hai.

  4. Index Kya Hai? Types of Indexes?
    Index ek data structure hai jo query performance improve karta hai. Types:

    • Clustered Index: Table ke data ko physically sort karta hai (ek table mein sirf ek hota hai).

    • Non-Clustered Index: Alag data structure mein pointers rakhta hai.

  5. Views Kya Hain? Yeh Kaise Useful Hain?
    View ek virtual table hai jo query ke result pe based hoti hai. Useful for:

    • Complex queries ko simplify karne ke liye.

    • Data security ke liye (specific columns show karne ke liye).

  6. IN, EXISTS, aur ANY Mein Kya Difference Hai?

    • IN: Multiple values ke saath compare karta hai.

    • EXISTS: Subquery ke result ke existence ko check karta hai.

    • ANY: Subquery ke kisi bhi value ke saath compare karta hai.

  7. Duplicate Rows Kaise Find Karoge?

    SELECT column_name, COUNT(*) 
FROM table_name 
GROUP BY column_name 
HAVING COUNT(*) > 1;

  8. Top N Records Kaise Fetch Karoge?

    • MySQL: SELECT * FROM table_name LIMIT N;

    • SQL Server: SELECT TOP N * FROM table_name;

    • Oracle: SELECT * FROM table_name WHERE ROWNUM <= N;

  9. Stored Procedure aur Function Mein Kya Difference Hai?

    • Stored Procedure: Multiple operations perform karta hai, return value optional.

    • Function: Hamesha ek value return karta hai, DML statements limited hote hain.

  10. Trigger Kya Hai?
    Trigger ek special stored procedure hai jo automatically events (INSERT, UPDATE, DELETE) pe run hota hai.

  11. ACID Properties Kya Hain?

    • Atomicity: Transaction completely execute ho ya fail ho.

    • Consistency: Database consistent state mein rahe.

    • Isolation: Transactions ek doosre se independent rahein.

    • Durability: Committed changes permanent ho.

  12. Constraints Kya Hain?
    Constraints rules hote hain jo data integrity maintain karte hain: PRIMARY KEY, FOREIGN KEY, NOT NULL, UNIQUE, CHECK.

  13. CHAR, VARCHAR, aur TEXT Mein Difference?

    • CHAR: Fixed length, padding ke saath.

    • VARCHAR: Variable length, no padding.

    • TEXT: Large text data ke liye, no fixed length.

✅ Advanced SQL Interview Questions

  1. Second Highest Salary Kaise Find Karoge?

    SELECT MAX(salary) 
FROM employees 
WHERE salary < (SELECT MAX(salary) FROM employees);

  2. Department with Highest Number of Employees Ka Query Likho.

    SELECT department_id, COUNT(*) as emp_count 
FROM employees 
GROUP BY department_id 
ORDER BY emp_count DESC 
LIMIT 1;

  3. CTE (Common Table Expression) Kya Hai?
    CTE ek temporary result set hai jo query ke execution ke liye use hota hai.

    WITH cte_name AS (SELECT * FROM table_name) 
SELECT * FROM cte_name;

  4. Window Functions Kya Hain?
    Window functions rows ke set pe calculations perform karte hain. Examples:

    • ROW_NUMBER(): Har row ko unique number assign karta hai.

    • RANK() / DENSE_RANK(): Ranking deta hai, ties ke liye alag logic.

    • LEAD() / LAG(): Next ya previous row ka data access karta hai.

  5. Slow SQL Query Ko Kaise Optimize Karoge?

    • Indexes create karo.

    • Unnecessary columns select mat karo.

    • Joins ko optimize karo.

    • EXPLAIN PLAN use karke query analyze karo.

  6. Clustered aur Non-Clustered Index Mein Difference?

    • Clustered: Data physically sorted hota hai (ek table mein ek hi hota hai).

    • Non-Clustered: Alag index structure, pointers ke saath.

  7. OLTP aur OLAP Mein Kya Difference Hai?

    • OLTP: Transaction-oriented, fast inserts/updates (e.g., banking systems).

    • OLAP: Analysis-oriented, complex queries (e.g., data warehouses).

  8. Materialized View Kya Hai?
    Materialized View query result ko physically store karta hai, unlike regular views jo virtual hote hain.

✅ Practical SQL Query Questions

  1. Highest Salary in Each Department:

    SELECT d.department_name, MAX(e.salary) 
FROM employees e 
JOIN departments d ON e.department_id = d.department_id 
GROUP BY d.department_name;

  2. Count Employees in Each Department:

    SELECT department_id, COUNT(*) 
FROM employees 
GROUP BY department_id;

  3. Employees Joined in Last 3 Months:

    SELECT * 
FROM employees 
WHERE hire_date >= DATEADD(MONTH, -3, GETDATE());

  4. Delete Duplicates from a Table:

    WITH cte AS (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY column_name ORDER BY column_name) as rn 
    FROM table_name
)
DELETE FROM cte WHERE rn > 1;

✅ Behavioral & Miscellaneous SQL Questions

  1. SQL Injection Kaise Handle Karoge?

    • Prepared statements ya parameterized queries use karo.

    • User input ko validate karo.

    • Avoid dynamic SQL with direct user input.

  2. Large Datasets Kaise Handle Karte Ho?

    • Indexing use karo.

    • Pagination implement karo.

    • Batch processing use karo for large updates.

  3. Database Migration Kaise Karoge?

    • Schema aur data backup lo.

    • Migration scripts likho.

    • Test environment mein pehle try karo.

  4. Backup aur Restore Kaise Karoge?

    • Backup: BACKUP DATABASE db_name TO DISK = 'path';

    • Restore: RESTORE DATABASE db_name FROM DISK = 'path';

Yeh questions aur answers aapko SQL interviews ke liye ready karne mein help karenge. Practice karo, queries likho, aur concepts clear rakho. All the best for your interview! 💪

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