SQL嵌套式查询作业SQL小提琴

Question

找到最大成员（患者）数量的保险公司。

这就是数据库和我第一次尝试更贴近：

http://sqlfiddle.com/#!3/4b2ed/27

SELECT P.cid AS cid, COUNT (P.pid) AS CountP 
FROM  Patient P 
GROUP BY P.cid 

如何获得最大countp？

Answer 1

只是为了你的结果降序的病例数，并只取第一条记录。

SELECT P.cid AS cid, COUNT(P.pid) AS CountP 
FROM  Patient P 
GROUP BY P.cid 
ORDER BY CountP DESC 
LIMIT 1 

如果不止一家保险公司可能有成员是最大的号码，你想获取所有的人，你将需要使用子查询：

SELECT P.cid AS cid, COUNT(P.pid) AS CountP 
FROM  Patient P 
GROUP BY P.cid 
HAVING  CountP = (
    SELECT COUNT(pid) AS c 
    FROM  Patient 
    GROUP BY cid 
    ORDER BY c DESC 
    LIMIT 1 
); 

在你描述的怪现状在您的意见，其中一个是无法使用LIMIT，你可以这样做：

SELECT P.cid AS cid, COUNT(P.pid) AS CountP 
FROM  Patient P 
GROUP BY P.cid 
HAVING  CountP = (SELECT MAX(d.c) FROM (
    SELECT COUNT(pid) AS c 
    FROM  Patient 
    GROUP BY cid 
) AS d); 

Answer 2

尝试用以下：
最大（计数（P.pid））

Answer 3

SELECT P.cid, COUNT (P.pid) AS CountP 
FROM Patient P 
GROUP BY P.cid 
compute max(count(p.pid)) 

Answer 4

哇 - 很多限制。在这里你去：

SELECT MAX(CountP) FROM (
    SELECT P.cid, COUNT (P.pid) AS CountP 
    FROM Patient P 
    GROUP BY P.cid 
) winner