Module 5 · From query to a defensible answer Lesson 14 of 15

Data quality checks you run every time

Worked example

SELECT email, COUNT(*) c FROM users GROUP BY email HAVING c > 1 finds duplicate accounts; a salary < 0 filter finds impossible values.

Your practice task

Write four check queries for one table and note how many rows each one flags.

Do this before moving on. Reading a solution teaches the shape of it; typing it teaches you.

Key takeaway

Before any number goes out, run the same four checks: duplicate keys, values outside their allowed set, dates in the future, and totals that do not match the source.

Almost there

15 lessons left of 15

Finish every lesson to unlock the certificate. Each one ends in a practice task — do them and the final project brief writes itself.

Next recommended course Interview DSA Crash Course