Database

Combining conditions in SELECT … WHERE, and NULL’s three-valued logic: any comparison with NULL is not true, so use IS NULL

Shard-key choice as workload design, not just picking a column

Database replication as a freshness and routing problem, not just read scaling

A practical backend view of ACID, anomalies, MVCC, locks, and retry behavior

Reason about composite indexes through predicates, ordering, projection, plan shape, and write cost

Index reasoning through page reads, access paths, selectivity, row fetches, and write tax

LeetCode 584 note: referee_id != 2 misses NULLs, since comparing with NULL is unknown; add IS NULL explicitly

LeetCode 1757 note: both conditions must hold, so filter with WHERE … AND …

LeetCode 620 note: odd id and description not ‘boring’, ordered by rating from high to low

LeetCode 596 note: group by class with HAVING COUNT(DISTINCT student) >= 5; older test data had duplicate rows, so count distinct students

LeetCode 595 note: filter with WHERE and OR for an area of at least 3 million or a population of at least 25 million

LeetCode 197 note: self-join Weather on a one-day date difference with today warmer; compare dates with DATEDIFF, never ids