联合索引的最左前缀原则，什么情况下会失效？

有索引 (a, b, c)，下面哪些查询能用上索引？ 1) where b = 1 2) where a = 1 and c = 2 3) where a = 1 and b > 2 and c = 3 4) where a = 1 order by b