list of 25 essential DBMS interview questions and answers for experienced professionals, covering fundamental concepts, advanced topics, and practical scenarios:
1. What is ACID in DBMS?
Answer:
ACID stands for:
- Atomicity: Transactions are all-or-nothing.
- Consistency: Database remains in a valid state before and after transactions.
- Isolation: Concurrent transactions don’t interfere.
- Durability: Committed transactions persist even after crashes.
2. Explain Normalization vs. Denormalization.
Answer:
- Normalization: Minimizes redundancy (1NF, 2NF, 3NF, BCNF).
- Denormalization: Improves read performance by adding redundancy (used in data warehouses).
3. What are Indexes? How do they improve performance?
Answer:
Indexes are data structures (e.g., B-trees) that speed up data retrieval. Trade-off: Slower writes due to index updates.
4. Compare SQL and NoSQL databases.
Answer:
| SQL (MySQL, PostgreSQL) | NoSQL (MongoDB, Cassandra) |
|---|---|
| Structured schema | Schema-less |
| Vertical scaling | Horizontal scaling |
| ACID compliant | BASE model (Basically Available, Soft state, Eventually consistent) |
5. What is a Deadlock? How to prevent it?
Answer:
- Deadlock: Two transactions block each other.
- Prevention: Use timeouts, lock ordering, or deadlock detection algorithms.
6. Explain CAP Theorem.
Answer:
A distributed DB can only guarantee 2 of these 3 properties:
- Consistency: All nodes see the same data.
- Availability: Every request gets a response.
- Partition Tolerance: System works despite network failures.
7. What is a Clustered vs. Non-Clustered Index?
Answer:
- Clustered: Reorders physical data (only 1 per table).
- Non-Clustered: Separate structure (multiple allowed).
8. How does a JOIN operation work internally?
Answer:
Common algorithms:
- Nested Loop: For small tables.
- Hash Join: For equality conditions.
- Merge Join: For sorted data.
9. What is Database Sharding?
Answer:
Splitting a database into smaller, faster pieces (shards) distributed across servers.
10. Explain OLTP vs. OLAP.
Answer:
| OLTP (Transactions) | OLAP (Analytics) |
|---|---|
| Short, frequent transactions | Complex queries |
| Normalized data | Denormalized data |
| e.g., Banking systems | e.g., Data warehouses |
11. What is a View? When would you use it?
Answer:
A virtual table based on a query. Uses:
- Simplify complex queries.
- Restrict data access (security).
12. How do you optimize a slow SQL query?
Answer:
- Add indexes.
- Avoid
SELECT *. - Use
EXPLAINto analyze the query plan.
13. What is a Transaction Isolation Level?
Answer:
Defines how transactions interact:
- Read Uncommitted: Lowest isolation, dirty reads.
- Read Committed: Default in most DBs.
- Repeatable Read: No dirty or non-repeatable reads.
- Serializable: Highest isolation.
14. Explain Database Replication.
Answer:
Maintaining multiple copies of data for:
- High availability.
- Faster read scaling (read from replicas).
15. What is a Stored Procedure? Pros and Cons?
Answer:
Precompiled SQL statements stored in the DB.
- Pros: Reusable, secure, reduces network traffic.
- Cons: Hard to debug, vendor-specific.
16. Compare MongoDB and PostgreSQL.
Answer:
| MongoDB | PostgreSQL |
|---|---|
| Document-based | Relational |
| Dynamic schema | Fixed schema |
| Horizontal scaling | Vertical scaling |
17. What is a Trigger?
Answer:
Automated actions (e.g., logging) triggered by DB events (insert/update/delete).
18. How does Database Caching work?
Answer:
Stores frequently accessed data in memory (e.g., Redis) to reduce disk I/O.
19. What is Connection Pooling?
Answer:
Reusing DB connections to reduce overhead of creating new ones.
20. Explain BASE Properties.
Answer:
Alternative to ACID for NoSQL:
- Basically Available: System responds even if degraded.
- Soft State: State may change over time.
- Eventually Consistent: Data becomes consistent over time.
21. What is a Materialized View?
Answer:
A precomputed view stored physically (used in data warehouses for faster queries).
22. How do you handle Database Migrations?
Answer:
Tools like Flyway or Liquibase to version-control schema changes.
23. What is a Composite Key?
Answer:
A primary key made of multiple columns (e.g., (student_id, course_id)).
24. Explain the Difference Between DELETE, TRUNCATE, and DROP.
Answer:
- DELETE: Removes rows (can rollback).
- TRUNCATE: Faster, removes all rows (no rollback).
- DROP: Deletes the entire table.
25. How do you ensure Database Security?
Answer:
- Encryption (at rest and in transit).
- Role-Based Access Control (RBAC).
- Regular audits.
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This list balances theory (ACID, CAP) with practical skills (query optimization, sharding) and real-world comparisons (SQL vs. NoSQL).
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