sql查询（三）--having子句求众数、中位数

一、建立需要查询的表
CREATE TABLE Graduates
(name   VARCHAR(16) PRIMARY KEY,
 income INTEGER NOT NULL);
-- 桑普森是个离群值，会拉高平均数
INSERT INTO Graduates VALUES('桑普森', 400000);
INSERT INTO Graduates VALUES('迈克',     30000);
INSERT INTO Graduates VALUES('怀特',   20000);
INSERT INTO Graduates VALUES('阿诺德', 20000);
INSERT INTO Graduates VALUES('史密斯',     20000);
INSERT INTO Graduates VALUES('劳伦斯',   15000);
INSERT INTO Graduates VALUES('哈德逊',   15000);
INSERT INTO Graduates VALUES('肯特',     10000);
INSERT INTO Graduates VALUES('贝克',   10000);
INSERT INTO Graduates VALUES('斯科特',   10000);

1、求众数
（1）在having子句中用包含谓词all 的子查询
SELECT income, COUNT(*) AS cnt 
FROM Graduates 
GROUP BY income HAVING COUNT(*) >= ALL ( SELECT COUNT(*) FROM Graduates GROUP BY income)；
缺点：谓词all 用于null和 空集时会出现问题
（2）在having子句中 用 包含极值函数的子查询
select income ,count(*) as cnt
from graduates
group by income
having cnt >= (select max(cnt)
　　　　　　　　　from (select count(*) as cnt
　　　　　　　　　　　　from graduates
　　　　　　　　　　　　group by income) as tmp));

2、求中位数
用having子句进行自连接求中位数
第一步-- 将集合里的元素按照大小分为上半部分、下班部分 两个子集，求其交集（无论聚合数据的数目是 奇数 偶数）
select  t1.income 
from gradutes t1 , gradutes t2
group by t1.income
having sum(case when t2.income >=t1.income then 1 else 0 end) >= count(*)/2
and sum(case when t2.income <=t1.income then 1 else 0 end) >= count(*)/2;
第二步 -- 将上下部分集合求得的交集，去重，然后求平均，得到中值
select avg(distinct income)
from ( select t1.income
　　　　from gradutes t1,gradutes t2
　　　　group by t1.income
　　　　having sum(case when t2.income >= t1.income then 1 else 0) >= count(*)/2
　　　　　　and sum (case when t2.incomme <= t1.income then 1 else 0 ) >= count(*)/2) as tmp

sql查询（三）--having子句求众数、中位数

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