ACID properties ensure reliable database transactions: Atomicity (all-or-nothing), Consistency (valid state), Isolation (concurrent independence), and Durability (permanent storage).

INNER JOIN returns only rows with matching values in both tables. LEFT JOIN includes unmatched left table rows, FULL OUTER JOIN includes all rows, and CROSS JOIN produces Cartesian product.

LRU cache requires O(1) access and O(1) removal/insertion of least-used items. Hash Map provides O(1) lookup, and Doubly Linked List maintains order efficiently.

Balanced BSTs (AVL, Red-Black) maintain O(log n) search time by keeping height logarithmic. Unbalanced trees degrade to O(n) in worst case.

IaaS provides virtualized computing resources over the internet (AWS EC2, Azure VMs). PaaS and SaaS are higher-level abstractions.

3NF removes transitive dependencies where non-key attributes depend on other non-key attributes. BCNF is stricter but 3NF eliminates transitive dependencies.

Replication maintains copies across nodes for high availability, disaster recovery, and load distribution. It doesn't eliminate backups—they serve different purposes.

Firewalls filter traffic based on security policies. While they provide protection, encryption is separate, antivirus is different, and authentication is handled by other systems.

Modern languages use Introsort which starts with Quicksort (O(n log n) avg) but switches to Heapsort (O(n log n) worst) to avoid worst-case scenarios.

CAP theorem (Brewer's theorem) states distributed systems must choose 2 of 3 properties: strong consistency (C), continuous availability (A), or tolerance to network partitions (P).