Updating, Deleting, and Knowing Whether It Happened

res, err := db.Exec(`UPDATE tasks SET done = ? WHERE id = ?`, true, id)

If no row has that id, err is nil. The statement ran, it was valid, it matched nothing, and SQL considers that a complete success. Your API almost certainly does not — a PATCH /tasks/999 should be a 404, and this is the only place you can tell.

n, err := res.RowsAffected()
if err != nil {
	return fmt.Errorf("update task %d: %w", id, err)
}
if n == 0 {
	return ErrNotFound
}

That check is the difference between an endpoint that reports what happened and one that cheerfully returns 204 for a task that never existed. The same applies to DELETE: deleting nothing is not an error, so RowsAffected() == 0 is how "there was nothing to delete" reaches the caller.

Zero rows affected is not always "not found"

One subtlety worth knowing before it confuses you. Some databases (MySQL, by default) report zero affected rows for an UPDATE that matched a row but changed nothing — setting done = true on a task already done. Under that behaviour, n == 0 conflates "no such row" with "no change needed", and a 404 would be wrong.

SQLite and PostgreSQL count matched rows, so n == 0 really does mean "no such row" there. If you need certainty across databases, check existence separately, or use RETURNING.

Exec for writes, always

Reaching for Query on an UPDATE still works and still leaks a connection. The rule doesn't change: statements that return rows use Query/QueryRow; statements that don't use Exec.

The pattern this completes

You now have the four operations the SQL-backed store needs, and each reports absence in the same vocabulary:

Operation	How "not there" arrives
Get one	sql.ErrNoRows from Scan
List	an empty slice — absence is not an error
Update	RowsAffected() == 0
Delete	RowsAffected() == 0

Three different mechanisms, one ErrNotFound at your boundary. That translation is what lets the layer above stay ignorant of SQL entirely — which is exactly what the next part builds.

Try it

Implement update and delete, and make both tell the caller when there was nothing to act on.