Query Systems · Knowledge Base
SQL
From first queries to advanced analytics. Joins, aggregation, and window functions; query reasoning and debugging; and the performance judgment to write SQL that stays correct — and cheap — at scale.
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3 chaptersSQL Essentials
Read a table and its grain, shape a query, select and filter deliberately, handle types and NULLs, count safely, and inspect results like an engineer.
Read chapter →Joins & Relationship Reasoning
Join types, grain, cardinality, duplicate explosions, and the habit of proving why rows multiplied.
Read chapter →Aggregation & Business Metrics
Grouped metrics, HAVING filters, distinct counts, conditional aggregation, and trustworthy metric definitions.
Read chapter →Medium
3 chaptersCTEs, Subqueries & Query Structure
Layered query design, debuggable intermediate results, and SQL that is easy to narrate in interviews.
Read chapter →Window Functions & Analytics Patterns
Ranking, offsets, running totals, deduplication, retention, cohorts, and event sequence analysis.
Read chapter →Time, Events & Semi-Structured Data
Timestamp logic, time zones, event tables, JSON, arrays, late data, and ordering tradeoffs.
Read chapter →Advanced
4 chaptersData Engineering SQL Usage
Incremental loads, MERGE patterns, upserts, slowly changing dimensions, snapshots, and backfills.
Read chapter →Data Quality, Debugging & Auditing
Duplicate checks, null checks, referential tests, reconciliation, anomaly queries, and audit evidence.
Read chapter →Warehouse Performance & Cost
Partitions, clustering, pruning, query plans, scan cost, and warehouse-aware optimization habits.
Read chapter →SQL Interview Playbook
Clarifying assumptions, narrating query logic, debugging live, and explaining senior-level tradeoffs.
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