SQL Query Explainer

In short Reading a SQL query means reading it in execution order rather than written order, and those differ. It is written SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY; it is evaluated FROM, WHERE, GROUP BY, HAVING, SELECT, ORDER BY. That explains two things beginners find confusing: why a column alias defined in SELECT cannot be used in WHERE, and why WHERE filters rows while HAVING filters groups. On performance, the patterns that cause most slow queries are recognisable without a profiler. A function applied to a column in WHERE prevents the index on it being used, so `WHERE YEAR(created) = 2026` scans where a range comparison would seek. `SELECT *` on a wide table moves data nobody reads. A LEFT JOIN followed by a WHERE condition on the right-hand table silently behaves as an INNER JOIN. And a join on columns of different types forces a conversion that also defeats the index. None of this replaces reading the actual execution plan, which is the only thing that tells you what the database really did.