The Library · Insights
Deep dives across data, automation & AI
108 in-depth articles — from Excel foundations to production machine learning. Filter by topic or difficulty to find exactly what you need.
SQLExpert
Writing SQL to Detect and Resolve Lock Contention: Using Query Plans, Wait Statistics, and Transaction Isolation to Diagnose and Fix Blocking in High-Concurrency Systems
Lock contention is one of the most damaging and misunderstood performance problems in production databases. This expert-level lesson walks you through the complete diagnostic and resolution workflow — from tracing blocking chains with recursive CTEs and system views, to reading execution plans for lock behavior, to choosing the right isolation level and implementing fixes that actually stick.
31 min read
SQLFoundation
Filtering Rows Before and After Aggregation: WHERE vs HAVING in GROUP BY Queries
Most SQL errors in grouped queries come from filtering at the wrong stage. This lesson teaches you exactly when to use WHERE versus HAVING — and what goes wrong when you mix them up.
15 min read
SQLPractitioner
Writing SQL to Simulate LAG and LEAD Across Partition Boundaries: Handling Edge Cases Where Window Functions Return NULL
Window functions like LAG and LEAD stop at partition boundaries by design — but your business logic often doesn't. This lesson teaches five concrete SQL techniques for reaching across partition boundaries, with realistic e-commerce examples and performance guidance for each approach.
22 min read
SQLExpert
Writing SQL to Detect and Fill Sequence Gaps: Using Self-Joins and Subqueries to Find Missing IDs and Date Ranges
Sequence gaps — missing IDs, uncovered date ranges, holes in attendance records — hide data quality problems that standard queries never surface. This expert-level lesson teaches you every production-ready technique for finding and filling them, from self-joins to window functions, with full performance analysis.
27 min read
SQLFoundation
Reading and Interpreting SQL Query Execution Plans: A Beginner's Guide to EXPLAIN Output and Scan Types
Slow queries are frustrating — but the database is already telling you exactly what's wrong. Learn to read SQL execution plans from scratch: understand EXPLAIN output, decode scan types like Seq Scan and Index Scan, and spot performance problems before they hit production.
17 min read
SQLPractitioner
Writing SQL to Calculate Period-Over-Period Change: Date Filtering, GROUP BY, and Derived Table Comparisons in One Query
Learn how to compare two time periods — month-over-month, quarter-over-quarter — in a single SQL query using derived tables, FULL OUTER JOIN, and CASE-based percentage change logic. Includes CTEs, conditional aggregation alternatives, and a hands-on exercise with realistic data.
20 min read
SQLExpert
Writing SQL for Slowly Changing Dimensions: Type 1, Type 2, and Type 6 Merge Patterns Using Window Functions and CTEs
Slowly Changing Dimensions are where data warehouse pipelines either earn trust or quietly corrupt history. This lesson teaches you production-grade SQL for Type 1 overwrites, Type 2 versioned history, and the hybrid Type 6 pattern — using MERGE, CTEs, and window functions to handle late-arriving data, phantom changes, and scale.
30 min read
SQLFoundation
Ordering and Limiting Query Results: Using ORDER BY, LIMIT, OFFSET, and TOP to Control What SQL Returns
SQL doesn't sort or trim your results unless you tell it to. Learn how ORDER BY, LIMIT, OFFSET, and TOP give you precise control over what your queries return — and in what order — across MySQL, PostgreSQL, and SQL Server.
16 min read
SQLPractitioner
Optimizing Window Function Performance: Partitioning Strategies, Frame Boundaries, and Index Alignment for Large Datasets
Window functions are powerful but can destroy query performance at scale. Learn how to design partition strategies, choose the right frame boundaries, and align indexes to eliminate sort spills and cut execution times by orders of magnitude.
25 min read
SQLExpert
Writing SQL to Detect and Handle Data Gaps: Using LEFT JOIN and Subqueries to Find Missing Records Across Related Tables
Data gaps are invisible by default — SQL only shows you what exists, not what's missing. This lesson teaches you how to systematically detect missing records across related tables using LEFT JOIN anti-joins, NOT EXISTS correlated subqueries, generated date spines, and FULL OUTER JOIN reconciliation patterns, with production-quality examples and performance guidance throughout.
29 min read
SQLFoundation
Understanding SQL Query Execution Plans: Reading EXPLAIN Output to Diagnose and Optimize Slow Queries
Stop guessing why your SQL queries are slow. Learn to read EXPLAIN and EXPLAIN ANALYZE output to diagnose performance problems, identify expensive operations, and apply targeted fixes that actually work.
19 min read
SQLPractitioner
Writing SQL to Find Duplicates and Deduplicate Records: GROUP BY, HAVING, and Subquery Strategies
Duplicate records silently corrupt every report and join that touches them — often without raising an error. Learn how to detect exact and partial duplicates with GROUP BY and HAVING, then safely remove them using ROW_NUMBER() subquery patterns you can trust with production data.
21 min read