ORDER BY with NULLs first or last

-- Modern engines.
SELECT * FROM users ORDER BY last_seen_at DESC NULLS LAST;
SELECT * FROM users ORDER BY signup_date    ASC  NULLS FIRST;

-- MySQL workaround (no NULLS clause).
SELECT * FROM users ORDER BY last_seen_at IS NULL, last_seen_at DESC;
-- "IS NULL" returns 0 for non-null and 1 for null; sorting by it puts nulls last.

-- SQL Server workaround (any version — no native NULLS clause).
SELECT * FROM users ORDER BY CASE WHEN last_seen_at IS NULL THEN 1 ELSE 0 END, last_seen_at DESC;

-- "Show users by last login, with users who never logged in last."
SELECT * FROM users ORDER BY last_login_at DESC NULLS LAST;

-- Pagination: sort key must be deterministic — use a tie-breaker.
SELECT * FROM orders
ORDER BY created_at DESC NULLS LAST, id DESC      -- id breaks ties + NULL ordering
LIMIT 50 OFFSET 0;

-- "Show items needing review (NULL fields) at the top."
SELECT * FROM tickets ORDER BY priority ASC NULLS FIRST, created_at;

-- Sort by computed field with explicit NULL handling.
SELECT *,
       (revenue / NULLIF(visitors, 0)) AS rev_per_visitor
FROM   campaigns
ORDER  BY rev_per_visitor DESC NULLS LAST;