Core SQL

Build your SQL foundation with logic, ordering, and null semantics that matter in production.

Foundations that trip people up

These pages cover the semantics most likely to cause bad production queries:

NULL handling

What NULL means, how comparisons behave, and how COALESCE helps.

WHERE

Filter rows before grouping, and see how NULL and join filters change the result.

ORDER BY

How ordering works, why ties matter, and how to make output deterministic.

GROUP BY

When aggregates change the shape of your result set.

CASE

Branch on conditions to reshape values for filtering, grouping, and display.

Keep this checklist close

NULL is not equal to anything
ORDER BY without a tiebreaker is not stable
GROUP BY changes the grain of the result
a query is only safe when the business rule is explicit

Example

SELECT
  customer_id,
  COALESCE(MAX(order_date), 'never') AS last_order_date,
  COUNT(*) AS orders
FROM orders
GROUP BY customer_id
ORDER BY customer_id;