1. 建立測試表及測試資料

CREATE TABLE graduates (
  name varchar(255) ,
  income int4(255) 
);

INSERT INTO graduates VALUES ('桑普森', '400000');
INSERT INTO graduates VALUES ('邁克', '30000');
INSERT INTO graduates VALUES ('懷特', '20000');
INSERT INTO graduates VALUES ('阿諾德', '20000');
INSERT INTO graduates VALUES ('史密斯', '20000');
INSERT INTO graduates VALUES ('勞倫斯', '15000');
INSERT INTO graduates VALUES ('哈德遜', '15000');
INSERT INTO graduates VALUES ('肯特', '10000');
INSERT INTO graduates VALUES ('貝克', '10000');
INSERT INTO graduates VALUES ('斯科特', '10000');
 

2. 取眾數

select income,count(*) cnt 
from graduates
group by income
having count(*) >= (
select max(cnt) from (select count(*) cnt from graduates group by income) tmp
);

3. 取中位數

select AVG(DISTINCT income)
from (
select T1.income from graduates T1,graduates 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
) tmp;