Data, SQL and Getting to the Answer

A first course in data for people who have never written a query. You will learn SELECT and WHERE, how to reason about joins instead of memorising diagrams, GROUP BY, the NULL rules that quietly break counts, what an index actually does, how to clean real exported data without destroying it, and how to tell a number that is technically correct from a number that answers the question you were asked. It ends with the part nobody teaches: how to get SQL out of an AI without being confidently misled.

1. 1When a spreadsheet stops working7 minA database stores each fact once and enforces rules; a spreadsheet stores copies and enforces nothing.
2. 2SELECT and WHERE: asking for rows7 minWHERE tests one row at a time, in isolation; it never sees the other rows that share a customer.
3. 3Joins: reason about them, do not memorise them9 minA join emits one row per match; zero matches and two matches are where every join bug lives.
4. 4GROUP BY: making piles and asking about each one7 minEach aggregate answers a question about one pile, and COUNT(*) and COUNT(DISTINCT) are different questions.
5. 5NULL, and why your counts are wrong8 minWHERE keeps only rows where the test is true, and every comparison with NULL is unknown.
6. 6Why a query is slow, and what an index does8 minAn index lets the database skip rows; anything that hides a column's raw value takes that ability away.
7. 7Cleaning real data without destroying it9 minKeep the raw data, clean it in a query you can re-run, and count every row you drop.
8. 8A right number and a useful number are not the same9 minA correct query answers the question you typed, which is rarely the question you were asked.
9. 9Asking an AI for SQL without being lied to8 minPaste your schema, ask for assumptions first, and check the number against something you already know.