RRB JE Computer Science & IT — Complete CBT-2 Study Guide

• Keys: Primary key (uniquely identifies each row, cannot be NULL), Candidate key (any column/set qualifying to be a primary key), Foreign key (references a primary key in another table, enforces referential integrity), Composite key (primary key made of 2+ columns together). • Normalization reduces data redundancy: 1NF (atomic values, no repeating groups), 2NF (1NF + no partial dependency on part of a composite key), 3NF (2NF + no transitive dependency of non-key attributes on other non-key attributes) — higher normal forms reduce update/insert/delete anomalies. • ACID properties of a transaction: Atomicity (all-or-nothing execution), Consistency (database moves between valid states only), Isolation (concurrent transactions don't interfere), Durability (committed changes survive system failure). • SQL command categories: DDL (Data Definition Language — CREATE, ALTER, DROP), DML (Data Manipulation Language — SELECT, INSERT, UPDATE, DELETE), DCL (Data Control Language — GRANT, REVOKE). • Joins: INNER JOIN (only matching rows from both tables), LEFT JOIN (all rows from left table + matched rows from right, NULL where no match), RIGHT JOIN (mirror of LEFT), FULL OUTER JOIN (all rows from both, matched where possible).

• OSI model has 7 layers (top to bottom): Application, Presentation, Session, Transport, Network, Data Link, Physical — mnemonic "All People Seem To Need Data Processing." • TCP vs UDP: TCP is connection-oriented, reliable (acknowledgment + retransmission), ordered delivery — used for web browsing/email; UDP is connectionless, faster, no delivery guarantee — used for streaming/gaming/DNS where speed matters more than perfect reliability. • IP addressing: IPv4 is 32-bit (4 octets, e.g. 192.168.1.1); Class A/B/C/D/E ranges differ by leading bits; private IP ranges (10.x.x.x, 172.16-31.x.x, 192.168.x.x) are not routable on the public internet. • Networking devices: Hub (broadcasts to all ports, no intelligence, Layer 1); Switch (forwards only to the destination MAC address port, Layer 2, more efficient); Router (forwards packets between different networks based on IP address, Layer 3). • DNS (Domain Name System) translates human-readable domain names to IP addresses; DHCP (Dynamic Host Configuration Protocol) automatically assigns IP addresses to devices joining a network.

• Process vs Thread: a process is an independent program in execution with its own memory space; a thread is a lightweight sub-unit of a process sharing the same memory space — context switching between threads is faster than between processes. • CPU scheduling algorithms: FCFS (First Come First Served — simple, can cause long wait for short jobs behind long ones, "convoy effect"), SJF (Shortest Job First — minimizes average waiting time but needs burst time prediction), Round Robin (time-sliced, fair, good for time-sharing systems), Priority scheduling (can cause starvation of low-priority jobs without aging). • Deadlock requires FOUR simultaneous conditions (Coffman conditions): Mutual Exclusion, Hold and Wait, No Preemption, Circular Wait — breaking any ONE of these prevents deadlock. • Memory management: Paging (divides memory into fixed-size frames, eliminates external fragmentation but can cause internal fragmentation); Segmentation (divides memory into variable-size logical segments matching program structure, can cause external fragmentation). • Virtual memory allows a program to use more memory than physically available RAM by swapping pages between RAM and disk — page fault occurs when a needed page isn't in RAM and must be fetched from disk.

• Data structures — time complexity for search: Array (O(n) linear search, O(log n) binary search IF sorted), Linked List (O(n), no random access), Hash Table (O(1) average case), Binary Search Tree (O(log n) average, O(n) worst case if unbalanced). • Stack (LIFO — Last In First Out, used in function call management/recursion, undo operations) vs Queue (FIFO — First In First Out, used in task scheduling, print spooling, BFS traversal). • Sorting algorithm complexities: Bubble/Selection/Insertion sort — O(n²) average; Merge sort — O(n log n) guaranteed, needs extra space; Quick sort — O(n log n) average but O(n²) worst case, in-place (no extra space needed). • OOP (Object-Oriented Programming) pillars: Encapsulation (bundling data + methods, hiding internal details), Inheritance (a class acquiring properties of a parent class), Polymorphism (same interface, different implementations — method overloading/overriding), Abstraction (exposing only essential features, hiding complexity). • Recursion requires a base case (stopping condition) to avoid infinite recursion/stack overflow — every recursive call must move measurably closer to that base case.