SQL & Databases
Query design, indexing, and the trade-offs behind relational data modeling.
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Concepts
SQL Fundamentals
SELECT, WHERE, and JOIN — retrieving and combining rows from relational tables.
Aggregations & GROUP BY
Collapsing many rows into one summary row per group — counts, sums, and averages — plus the HAVING clause that filters groups.
Window Functions
Per-row calculations across a related set of rows — running totals, rankings, and row-over-row comparisons — without collapsing rows like GROUP BY does.
Schema Design & Normalization
Structuring tables to avoid redundant, inconsistent data — and knowing when to deliberately break the rules for performance.
Indexing & Query Performance
Why some queries are instant and others scan the whole table — and how an index (usually a B-tree) changes that.
Transactions & Isolation Levels
ACID guarantees, and the isolation-level trade-off between correctness and concurrent throughput.