L2Original starter · Free

SQL queries, groups and joins

Part of the Business Analytics learning and recruiting path. Read the explanation and example, attempt the task, then open the self-check.

Know what to learn for your role.

From foundations to a work sample, with sources, tasks and self-checks.

Business and data analysisData engineering
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Understand

SELECT chooses columns, WHERE filters rows, GROUP BY aggregates and JOIN connects keys. Check key uniqueness before joins to avoid multiplying amounts. NULL is not numeric zero.

Worked example

Joining order A worth 100 to two payment attempts creates two rows. Summing 200 is duplication, not revenue growth.

Examples and numbers are educational, not employer questions, live market data or hiring criteria.

Practise and keep your output

Query monthly unique orders and revenue, join customers, then compare counts and totals before and after.

Keep inputs and sources, assumptions, steps, results and uncertainties in your own document. Attempt independently before checking below.

After attempting: answer and self-check

Check cardinality and keys; aggregate first where needed. COUNT(*) and COUNT(column) can differ with NULLs.

Official channels and further reading

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