mysql 找到topN 大 数字

找到第N大的数字， 如果没有就显示为NULL

SELECT DISTINCT(SecondHighestSalary) 
FROM 
(SELECT 
CASE
WHEN (SELECT (SELECT COUNT(*) FROM (SELECT DISTINCT(salary) FROM employee) t) < 88) THEN NULL
ELSE (SELECT MIN(salary) FROM (SELECT salary FROM employee a ORDER BY salary DESC LIMIT 0, 88) a) 
END SecondHighestSalary	
FROM employee) c

测试脚本如下：

create table `employee` (
	`id` int (11),
	`salary` int (11)
); 
insert into `employee` (`id`, `salary`) values('12','761');
insert into `employee` (`id`, `salary`) values('6','1037');
insert into `employee` (`id`, `salary`) values('87','38');
insert into `employee` (`id`, `salary`) values('1','786');
insert into `employee` (`id`, `salary`) values('39','1762');
insert into `employee` (`id`, `salary`) values('65','133');
insert into `employee` (`id`, `salary`) values('25','2529');
insert into `employee` (`id`, `salary`) values('96','403');
insert into `employee` (`id`, `salary`) values('8','4333');
insert into `employee` (`id`, `salary`) values('11','3164');
insert into `employee` (`id`, `salary`) values('5','831');
insert into `employee` (`id`, `salary`) values('18','3579');
insert into `employee` (`id`, `salary`) values('66','4778');
insert into `employee` (`id`, `salary`) values('26','4826');
insert into `employee` (`id`, `salary`) values('82','4957');
insert into `employee` (`id`, `salary`) values('63','1288');
insert into `employee` (`id`, `salary`) values('64','2711');
insert into `employee` (`id`, `salary`) values('73','1842');
insert into `employee` (`id`, `salary`) values('2','2627');
insert into `employee` (`id`, `salary`) values('78','1908');
insert into `employee` (`id`, `salary`) values('22','4667');
insert into `employee` (`id`, `salary`) values('77','199');
insert into `employee` (`id`, `salary`) values('69','576');
insert into `employee` (`id`, `salary`) values('32','2638');
insert into `employee` (`id`, `salary`) values('79','510');
insert into `employee` (`id`, `salary`) values('23','622');
insert into `employee` (`id`, `salary`) values('43','2491');
insert into `employee` (`id`, `salary`) values('44','4435');
insert into `employee` (`id`, `salary`) values('28','4552');
insert into `employee` (`id`, `salary`) values('86','4646');
insert into `employee` (`id`, `salary`) values('57','451');
insert into `employee` (`id`, `salary`) values('13','1852');
insert into `employee` (`id`, `salary`) values('62','2835');
insert into `employee` (`id`, `salary`) values('72','544');
insert into `employee` (`id`, `salary`) values('37','2896');
insert into `employee` (`id`, `salary`) values('89','4699');
insert into `employee` (`id`, `salary`) values('71','220');
insert into `employee` (`id`, `salary`) values('46','2333');
insert into `employee` (`id`, `salary`) values('95','2261');
insert into `employee` (`id`, `salary`) values('48','3210');
insert into `employee` (`id`, `salary`) values('17','398');
insert into `employee` (`id`, `salary`) values('20','57');
insert into `employee` (`id`, `salary`) values('74','176');
insert into `employee` (`id`, `salary`) values('54','4272');
insert into `employee` (`id`, `salary`) values('35','2172');
insert into `employee` (`id`, `salary`) values('84','4686');
insert into `employee` (`id`, `salary`) values('42','2595');
insert into `employee` (`id`, `salary`) values('76','3631');
insert into `employee` (`id`, `salary`) values('45','4022');
insert into `employee` (`id`, `salary`) values('68','3248');
insert into `employee` (`id`, `salary`) values('40','571');
insert into `employee` (`id`, `salary`) values('14','4094');
insert into `employee` (`id`, `salary`) values('51','3272');
insert into `employee` (`id`, `salary`) values('97','2057');
insert into `employee` (`id`, `salary`) values('30','2505');
insert into `employee` (`id`, `salary`) values('88','3805');
insert into `employee` (`id`, `salary`) values('55','4926');
insert into `employee` (`id`, `salary`) values('81','3423');
insert into `employee` (`id`, `salary`) values('7','811');
insert into `employee` (`id`, `salary`) values('52','4664');
insert into `employee` (`id`, `salary`) values('33','1842');
insert into `employee` (`id`, `salary`) values('41','4645');
insert into `employee` (`id`, `salary`) values('50','1490');
insert into `employee` (`id`, `salary`) values('24','3361');
insert into `employee` (`id`, `salary`) values('59','4504');
insert into `employee` (`id`, `salary`) values('99','805');
insert into `employee` (`id`, `salary`) values('67','4561');
insert into `employee` (`id`, `salary`) values('61','4641');
insert into `employee` (`id`, `salary`) values('38','2542');
insert into `employee` (`id`, `salary`) values('94','4361');
insert into `employee` (`id`, `salary`) values('15','1652');
insert into `employee` (`id`, `salary`) values('70','1903');
insert into `employee` (`id`, `salary`) values('10','2109');
insert into `employee` (`id`, `salary`) values('49','1428');
insert into `employee` (`id`, `salary`) values('29','558');
insert into `employee` (`id`, `salary`) values('60','708');
insert into `employee` (`id`, `salary`) values('3','1717');
insert into `employee` (`id`, `salary`) values('0','3162');
insert into `employee` (`id`, `salary`) values('75','1450');
insert into `employee` (`id`, `salary`) values('83','2333');
insert into `employee` (`id`, `salary`) values('47','455');
insert into `employee` (`id`, `salary`) values('9','2360');
insert into `employee` (`id`, `salary`) values('56','1570');
insert into `employee` (`id`, `salary`) values('80','1677');
insert into `employee` (`id`, `salary`) values('27','508');
insert into `employee` (`id`, `salary`) values('21','3994');
insert into `employee` (`id`, `salary`) values('85','2276');
insert into `employee` (`id`, `salary`) values('91','1606');
insert into `employee` (`id`, `salary`) values('90','2269');
insert into `employee` (`id`, `salary`) values('58','1954');
insert into `employee` (`id`, `salary`) values('19','774');
insert into `employee` (`id`, `salary`) values('4','655');
insert into `employee` (`id`, `salary`) values('92','3274');
insert into `employee` (`id`, `salary`) values('16','333');
insert into `employee` (`id`, `salary`) values('36','1744');
insert into `employee` (`id`, `salary`) values('53','2262');
insert into `employee` (`id`, `salary`) values('93','3887');
insert into `employee` (`id`, `salary`) values('31','3521');
insert into `employee` (`id`, `salary`) values('34','1415');
insert into `employee` (`id`, `salary`) values('98','2490');

结果是： 510