Pandas, SQL, Power BI, Tableau, data visualization
41 articles
Does pandas.read_excel() seem to skip rows or load incorrect column names? In many Excel files, the actual header isn't on the first row because of titles, merged cells, or metadata. Learn why this happens, how the header parameter works, and the best ways to import Excel data accurately.
Your Pandas time series suddenly fills with NaN values after calling resample() or asfreq(). In most cases, Pandas isn't losing data—it is exposing missing timestamps in your dataset. Learn why these gaps appear and how to handle them correctly for reliable time series analysis.
Your pandas.merge() operation suddenly doubles or triples the number of rows, even though the merge completes successfully. The cause is usually non-unique join keys that produce many-to-many relationships. Learn why duplicate rows appear after merges and how to validate, diagnose, and fix them
Your HAVING clause looks correct but keeps dropping groups you know should be there. This guide breaks down every common reason HAVING silently eliminates valid rows — NULLs, type mismatches, WHERE vs HAVING confusion — and shows you exactly how to fix each one.
Your CASE WHEN logic looks correct, but the column keeps coming back NULL. This happens more often than you'd expect, and the root causes are almost always subtle — mismatched types, NULL comparison traps, or a missing ELSE clause silently swallowing rows.
Your Tableau dashboard looks complete, but certain rows keep disappearing without any error. If your data spans multiple time zones, a quiet UTC offset mismatch in date filters is likely the culprit. Here's how to catch and fix it.
Most data analysts use Jupyter Notebooks for one-off projects, but the same notebooks can become the foundation of a profitable recurring reporting business. Learn how to automate notebook execution, build client-ready reports, and transform your analytics expertise into a scalable subscription
Many data analysts build powerful Jupyter Notebook reports for internal projects, but few realize these notebooks can become profitable reporting services. Learn how to transform one-off analyses into scalable products, automate report generation, and build a recurring revenue business around data
A Tableau Top N filter often returns unexpected results because filters are evaluated in a specific order. If Context Filters are missing or configured incorrectly, Top N calculations may be performed against the entire dataset instead of the filtered subset. Learn how Tableau's order of operations
Your SQL query returns different row counts depending on which table you JOIN first — and no error is thrown. This guide breaks down exactly why JOIN order changes your results and how to fix it for good.
You ran the same query twice — once with DISTINCT and once with GROUP BY — and got different row counts. This guide breaks down exactly why that happens and how to get the result you actually want.
Power BI's built-in time intelligence functions assume a standard Gregorian calendar. When your data uses fiscal years, 4-4-5 periods, or custom date tables, those functions silently return wrong numbers. Here's how to diagnose and fix it.