CS Core Topics for Placement Interviews: DBMS, OS, Networks & OOP
A focused revision guide for CS core placement interview topics: DBMS, Operating Systems, Computer Networks and OOP — with the most frequently asked questions and answers.
Almost every technical interview in campus placements tests the same four areas: Database Management Systems, Operating Systems, Computer Networks and Object-Oriented Programming. These are the non-negotiables.
This guide gives you a focused revision map for each area — the sub-topics that appear most often, the style of questions interviewers ask, and the depth of answer that impresses versus the shallow recall that does not.
Key takeaways
- Four CS core pillars: DBMS, OS, Computer Networks, OOP. All four are tested in most technical interviews.
- DBMS: know keys, normalization to 3NF, all join types, SQL aggregation, ACID properties.
- OS: process vs thread, deadlock conditions, scheduling algorithms and synchronization are the most-asked clusters.
- Networks: OSI layers, TCP vs UDP, HTTP vs HTTPS, three-way handshake and DNS end-to-end.
- OOP: four pillars with examples, polymorphism types, abstract class vs interface.
- Prepare verbal answers out loud — technical interviews test explanation, not recall.
1. DBMS
DBMS questions are almost universally asked in technical interviews. The high-frequency sub-topics are:
- Keys: primary, candidate, foreign, super. Know the exact definition and the difference between candidate and primary key.
- Normalization: 1NF, 2NF, 3NF and BCNF. Be ready to normalize a given schema to 3NF step by step.
- Joins: inner, left, right, full outer. Know what each returns with an example.
- SQL: GROUP BY, HAVING vs WHERE, aggregate functions (COUNT, SUM, AVG, MAX, MIN), subqueries.
- Transactions: ACID properties. Be able to explain each property with an example.
- Indexing: what an index is, the trade-off (faster reads, slower writes), clustered vs non-clustered.
Interviewers probe depth: they start broad ('tell me about normalization') and then probe ('what anomalies does 2NF fix that 1NF does not?'). Prepare example-led answers, not just definitions.
2. Operating Systems
OS questions focus on three clusters:
- Process vs Thread: what is the difference, why threads share memory, when to use which. This is asked in nearly every service company interview.
- Deadlock: what it is, the four Coffman conditions (mutual exclusion, hold and wait, no preemption, circular wait), and prevention/avoidance strategies.
- Memory management: paging vs segmentation, virtual memory, page faults, thrashing.
- Scheduling algorithms: FCFS, SJF, Round Robin, Priority. Be able to compute average waiting time for each.
- Synchronization: semaphores, mutex, race conditions, the producer-consumer problem.
When asked 'what is a deadlock', do not just define it — describe a real scenario. The concrete example shows understanding, the definition shows memory.
3. Computer Networks
- OSI model: all 7 layers, what each does, which protocols live where.
- TCP vs UDP: connection-oriented vs connectionless, use cases, why UDP is used for video streaming.
- HTTP vs HTTPS: what HTTPS adds (TLS/SSL), why it matters.
- IP addressing: IPv4 vs IPv6, subnetting, the difference between public and private IP.
- DNS: what it does, the resolution process end to end.
- Three-way handshake: SYN, SYN-ACK, ACK — draw this from memory.
Networks questions are often lateral: 'what happens when you type a URL in a browser?' covers DNS, TCP, HTTP and rendering in one question. Practise narrating full end-to-end flows, not isolated facts.
4. OOP
OOP is often tested first because it underpins the recruiter's view of your programming foundation. The four pillars:
- 1Encapsulation: binding data and behaviour, access modifiers. Example: a BankAccount class that hides balance behind deposit/withdraw methods.
- 2Inheritance: parent-child class relationships, 'is-a' relationship, constructor chaining. Know why multiple inheritance is avoided in Java.
- 3Polymorphism: compile-time (method overloading) vs runtime (method overriding). The difference matters — interviewers ask it directly.
- 4Abstraction: hiding implementation, abstract classes vs interfaces. Know when to choose each.
Also revise: constructor vs destructor, static vs instance methods, virtual functions (C++), access specifiers (public, private, protected). In Java-focused interviews: String pool, == vs .equals(), garbage collection basics.
5. How to prepare in the right order
Prioritise by frequency of appearance: OOP first (asked most widely), then DBMS (asked in service and product companies), then OS, then Networks. Spend two days minimum on each area building depth on the sub-topics above.
After initial study, do verbal mock answers out loud. Explain 'what is normalization?' to yourself as if to an interviewer. If you stutter or skip steps, revisit. The gap between 'I know this' and 'I can explain this clearly' is exactly what technical interviews expose.
Frequently asked questions
How much depth do I need in CS core for service company interviews?
For service companies (TCS, Infosys, Wipro, Cognizant, Accenture), you need moderate depth: correct definitions, one clear example per concept, and the ability to distinguish similar terms (process vs thread, 2NF vs 3NF, TCP vs UDP). You do not need to implement OS schedulers or write SQL query optimizers.
Is DBMS asked in every placement technical interview?
DBMS is asked in almost every service company technical interview and in most product company interviews at the fresher level. It is the highest-frequency CS core subject. Normalize it to 3NF in your own words before your drive.
What is the difference between abstract class and interface in Java?
An abstract class can have both abstract and non-abstract methods, instance variables and constructors. An interface (pre-Java 8) can only have abstract methods. Post-Java 8, interfaces can have default and static methods. Use abstract class when you want a partial implementation; use interface when you want to define a contract that unrelated classes can implement.
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