One page.
One concept.
The reference sheets we post every day, in full. Read them here or grab the print-ready PDF, free, no signup.
Data Analyst
SQL window functions: why does GROUP BY reject your column?
A value per row, computed over a group, without collapsing anything. The anatomy, the ranking trio, and the top-N interview pattern.
SQL JOINs explained: why does your query return duplicate rows?
The 4 joins you actually use, plus the 2 traps that silently produce wrong numbers. Free one-page PDF included.
Why does your SQL alias error? The execution order
You write queries top to bottom. The engine runs them in a different order. That one fact explains the alias error, WHERE vs HAVING, and more.
Which chart should you use? The decision grid
Nine questions, nine answers. Pick charts by what you are asking the data, not by what looks impressive. Free print-ready PDF.
Data Scientist
Precision vs recall vs F1: which metric should you use?
Two questions, two metrics, one tradeoff. Plus the class-imbalance trap that makes accuracy meaningless. Free one-page PDF.
Which pandas functions do you actually need? The 8 verbs
Filter, select, sort, transform, group, aggregate, join, reshape. The pandas core in one page, with a free print-ready PDF.
Overfitting explained: why does your model fail the test set?
The difference between learning the pattern and memorizing the noise, how to diagnose which one your model did, and the five fixes.
Confusion matrix explained: which model mistake costs you money?
Rows, columns, the diagonal, the two error types, and the four formulas that fall out of one small grid. Free one-page PDF.
Data Engineer
Star schema vs snowflake: which one should you use?
The two classic warehouse shapes, when each one wins, and the vocabulary interviewers expect. Diagrams included, plus a free PDF.
OLTP vs OLAP: why is your dashboard slowing production?
Why the database that runs your app is the wrong place for your dashboards. Rows vs columns, the flow, and the prod-DB trap.
ETL vs ELT: which one should you choose?
Where the Transform step runs decides your tooling, your costs, and who gets to shape the data. The full comparison, plus a free PDF.
Business Analyst
What makes a good user story? The three-line anatomy
The three-line template, the INVEST test, and acceptance criteria that make developers say thank you. Free one-page PDF.
What makes a KPI actually useful? The SMART test
The SMART test, the difference between a KPI and a metric, and the red flags that mean your KPI is decoration. Free one-page PDF.
The 5 Whys method: how do you find a root cause?
The cheapest root-cause tool there is: a worked example, the rules that keep it honest, and the trap that ruins it.