For those, who cannot use MySQL 5.5.6+ and don’t want to write a stored procedure, there is another variant. We can add where clause on a subselect with ROWNUM.
SET @limit = 10;
SELECT * FROM (
SELECT instances.*,
@rownum := @rownum + 1 AS rank
FROM instances,
(SELECT @rownum := 0) r
) d WHERE rank < @limit;