Technical

The Query Detective

Learn to read SQL and work out exactly what a query returns — then prove it by cracking four locked boxes where a slightly wrong query gives a wrong code.

Intermediate
60
mins Certificate included
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The Query Detective

Course overview

About this course

Every number on every dashboard came from a query — and a great many of them are quietly wrong. A filter that excluded the wrong rows, a join that dropped records, a total that double-counted. This course teaches you to read SQL and work out exactly what a query returns from a set of data, so you can look at a reported figure and know whether to trust it.

No SQL writing required. You learn to read it like a detective — carefully, sceptically, exactly. Four locked boxes, each holding the next challenge, each opened by resolving a query correctly against dense data. Get the query slightly wrong and you get the wrong code.

What you'll learn

Practical outcomes

  • How to read SQL queries without writing them, and work out exactly what they return.
  • How filtering really works — including the NULL trap that quietly breaks real-world queries.
  • How joins drop rows (inner) or keep them (left), and how one-to-many joins inflate totals.
  • How aggregates and GROUP BY collapse data, and how HAVING filters groups.
  • The fixed order a database resolves a query in — and how to walk any query through it to get the exact result.

Pre-requisites

What you should know first

  • No SQL or coding experience required — every concept is taught from scratch.
  • Comfort reading tables of data and being precise with numbers will help.

Ready to begin?

Start learning through real challenges

Work through interactive exercises, unlock the next steps as you progress, and earn a certificate on completion.

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