SKILL
SQL JOIN & Aggregation
Join tables and answer business questions with GROUP BY
Beginner
SELECT plus WHERE plus ORDER BY on one table
Intermediate
INNER/LEFT JOIN plus GROUP BY plus HAVING for simple cohorts and rankings
Advanced
CTEs, window functions (ROW_NUMBER, LAG), slow-query optimization
Practice task
From the orders and customers tables (public dataset): write a query for top 10 customers by revenue last month using JOIN, GROUP BY, HAVING and sorting — include the query, the result, and one business insight sentence
Pass test
Pass test: send the query and result to 1 data peer. You pass if it joins at least 2 tables, the top 10 figures match a manual check of one row, the aggregation clause is correct, and you can explain why revenue is a sum of amounts rather than a count of orders.
Portfolio evidence
Commented .sql file plus result screenshot plus 3 lines of insight
Common mistake
LEFT JOIN silently becoming INNER through WHERE filters on NULLs; duplicated rows from non-unique join keys
USED IN THESE ROLES
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