Pandas, SQL, Power BI, Tableau, data visualization
28 articles
Three SQL window functions, one confusing result set. If your rankings are skipping numbers or repeating unexpectedly, you're probably using the wrong function for the job. Here's exactly how each one works and when to reach for it.
Reshaping DataFrames with melt or stack should be straightforward — until you end up with twice as many rows as expected. Here's what actually causes those duplicates and exactly how to fix them.
Your Top N filter in Tableau is supposed to show the top 10 customers by sales — but it keeps returning odd results that don't match expectations. The culprit is almost always context filters being ignored. Here's exactly how to fix it.
If your WHERE clause filters on NULL and keeps returning zero rows, you've hit one of SQL's oldest traps. The = NULL comparison silently fails every time — here's exactly why, and how to fix it for good.
Your DataFrame looks right but the numbers are wrong, or you're pulling yesterday's data without knowing it. Here's why pandas read_sql silently serves stale results and how to fix it for good.
DISTINCT and GROUP BY look interchangeable until your row counts stop making sense. This guide exposes the subtle traps each one sets and shows you exactly when to use which — with real query examples you can run today.
Calling resample() or asfreq() on your time series and getting a column full of NaNs? The fix is rarely where you expect it. This guide explains exactly why gaps appear and how to fill them correctly.
Your slicer filters one visual but leaves another completely untouched — and the data model looks fine. Cross-filter direction is usually the culprit, and fixing it is less obvious than it should be. Here's exactly what's going wrong and how to correct it.
Your SQL date filter looks right but the results are off — missing rows from the last day, pulling data from the wrong timezone, or silently truncating timestamps. Here's what's actually happening and how to fix it.
Your Tableau calculated field keeps returning null and you have no idea why. Nine times out of ten it's a type mismatch or an aggregation level issue — here's how to diagnose and fix both without guessing.
Using WHERE when you meant HAVING (or vice versa) produces silently wrong results — no errors, just bad numbers. Learn how to diagnose and fix the most common SQL aggregation filter bugs before they reach a dashboard.
You run the same logic as a CTE and as a subquery and get different row counts back. It's one of the most confusing bugs in SQL — here's exactly why it happens and how to fix it.