SQL
Queries, joins, indexes, transactions — with a live playground.
0/8 complete
- Querying DataSELECT, WHERE, ORDER BY, and GROUP BY — ask questions of a relational database, live.easy
- JoinsCombine rows from multiple tables on a shared key — the heart of relational databases.medium
- Indexes & PerformanceHow to make queries fast — indexes, execution plans, and avoiding the dreaded full table scan.medium
- Aggregation & GROUP BYCollapse many rows into per-group summaries with COUNT, SUM, AVG, MIN, MAX — then filter the groups with HAVING.easy
- Subqueries & CTEsNest a query inside another, then refactor it into readable named steps with WITH common table expressions.medium
- Window FunctionsCompute rankings, running totals, and row-to-row deltas without collapsing rows, using OVER (PARTITION BY ... ORDER BY ...).medium
- Transactions & IsolationACID guarantees, BEGIN/COMMIT/ROLLBACK, and how isolation levels rule out dirty, non-repeatable, and phantom reads.hard
- NormalizationDecompose a redundant table into 1NF, 2NF, and 3NF to kill update anomalies and store every fact exactly once.medium