Practical SQL examples, query patterns, and tips for data analysts and developers.
Find gaps in date sequences — and fill them with zero rows — using generate_series, recursive CTEs, and calendar tables. Works in PostgreSQL, MySQL 8, and BigQuery.
EXISTS and IN often return identical results with identical plans — but the NULL trap and correlated-subquery cases change everything. Here is when to pick which, with execution-plan notes.
The three ranking functions differ exactly when values tie — and that edge case is where leaderboards and top-N reports break. Side-by-side output table, plus when to use each one.
Write a SQL query to calculate month-over-month growth rate using window functions. Includes LAG(), CTE approach, and handling NULL months.
Step-by-step guide to joining 3+ tables in SQL. Covers INNER JOIN, LEFT JOIN ordering, and common mistakes that cause duplicate rows.
Two approaches to cumulative sums in SQL — the modern SUM() OVER() window function and the legacy self-join. With MySQL, PostgreSQL, and BigQuery examples.
How to identify duplicates with GROUP BY HAVING, and safely delete them using ROW_NUMBER() or a CTE. Works in MySQL, PostgreSQL, and SQL Server.
How text-to-SQL tools work under the hood — prompt engineering, schema awareness, and accuracy benchmarks. Plus when AI-generated SQL needs human review.
Get the most recent row for each group in SQL. Compares ROW_NUMBER(), correlated subquery, and DISTINCT ON approaches across MySQL, PostgreSQL, and BigQuery.
Count rows that meet specific conditions using CASE WHEN, FILTER, and IF. Includes counting distinct values with conditions and pivot-style summaries.
Update a table using values from another table. Covers MySQL, PostgreSQL, and SQL Server syntax differences with real examples.
Filtering by date is where beginners and pros diverge. Learn the exact WHERE clauses for today, yesterday, last 7 days, and current month across MySQL, PostgreSQL, and SQL Server.
CTEs and subqueries often produce identical results — but not identical plans. Here's when a WITH clause actually helps, when it hurts, and how to test it on your own database.
Turn category rows into columns with conditional aggregation — the portable technique that works on MySQL, PostgreSQL, SQLite, and SQL Server. Includes dynamic pivots.
Window functions are the most powerful SQL feature beginners skip. This guide explains OVER(), PARTITION BY, and ORDER BY with practical examples — row numbering, rankings, moving averages, and differences from GROUP BY.
Recursive CTEs explained with five practical examples: org charts, bill-of-materials, date range expansion, Fibonacci, and graph traversal. Includes the anchor/recursive structure and infinite-loop safeguards.
Why your SQL is slow and how indexes fix it: the index fundamentals, when indexes hurt, how to read EXPLAIN output, and the exact index designs for the most common slow-query patterns.
Write SQL to compare this year vs last year by month: YoY growth rate with LAG(), handling missing periods, and a ready-to-adapt CTE template.
Concatenate multiple rows into one string in SQL: MySQL GROUP_CONCAT, PostgreSQL STRING_AGG, SQL Server STRING_AGG, Oracle LISTAGG — with separators and ordering.
Get the full row that contains the max value per group in SQL: ROW_NUMBER() window function, NOT EXISTS, and a self-join — with MySQL and PostgreSQL examples.
The PostgreSQL vs MySQL syntax differences that actually bite: string concat, INTERVAL math, upserts, LIMIT argument order, JSON operators, regex, and identifier quoting — in one side-by-side table.
Every date operation you actually write — now, truncate, add intervals, diff, format, parse, end-of-month — mapped across four dialects in one table, plus the week-numbering and timezone traps.
NULL in SQL is three-valued logic — and it breaks NOT IN, COUNT, GROUP BY, and unique constraints in ways that look like data bugs. One reference table plus the eight traps that matter.
Stop guessing between INNER, LEFT, RIGHT, FULL, CROSS, and semi/anti joins. Two questions decide it every time — plus the WHERE-clause mistake that silently turns your LEFT JOIN into an INNER.
A decision guide for indexes: the three signals that justify one, the four cases where adding one slows writes for nothing, composite column order, and how to verify with EXPLAIN that the index is actually used.
Work through slow queries like a professional: read the plan first, fix predicates, then indexes, then rewrites — a numbered checklist that prevents the common mistake of tuning by guesswork.
Both are cloud warehouses with SQL — and the dialects diverge exactly where migration hurts: types, dates, arrays, QUALIFY, LIMIT vs FETCH, and the cost model that should shape your query style.
Eight SQL clients compared on price, platform support, and the features that matter daily — autocomplete, plan visualization, and multi-database support — with an honest note on who each one is wrong for.
A hands-on comparison of AI SQL generation approaches — dedicated tools vs general chatbots vs schema-aware generators — evaluated on the failure modes that matter: wrong joins, hallucinated columns, and dialect drift.
Twenty SQL interview questions that keep appearing in 2026 — joins, window functions, NULLs, indexing, and query design — each with the answer that gets you to the next round, not just the syntax.
Concatenation, trimming, substring, split-to-rows, regex replace, and padding — the string operations you write weekly, mapped with working syntax for PostgreSQL, MySQL, BigQuery, and SQL Server.
The 16 SQL errors you actually hit — from "column must appear in the GROUP BY clause" to deadlocks — organized by error message, with the cause and the fix for each.
Ten window-function patterns you will use for years — running totals, moving averages, deduplication, top-N per group, gap detection, sessionization, and percent-of-total — each as a copy-ready query.