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 cheat sheet
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 visually
The 4 joins you actually use, plus the 2 traps that silently produce wrong numbers. Free one-page PDF included.
Why your SQL alias errors: 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 for which data: 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, finally clear
Two questions, two metrics, one tradeoff. Plus the class-imbalance trap that makes accuracy meaningless. Free one-page PDF.
pandas: the 8 verbs that do 90% of the work
Filter, select, sort, transform, group, aggregate, join, reshape. The pandas core in one page, with a free print-ready PDF.
Overfitting vs underfitting, explained in three curves
The difference between learning the pattern and memorizing the noise, how to diagnose which one your model did, and the five fixes.
The confusion matrix, decoded
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 schema
The two classic warehouse shapes, when each one wins, and the vocabulary interviewers expect. Diagrams included, plus a free PDF.
OLTP vs OLAP: the one-image difference
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 and why
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
The anatomy of a good user story
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: root-cause analysis that actually works
The cheapest root-cause tool there is: a worked example, the rules that keep it honest, and the trap that ruins it.