mysql - SQL 嵌套查询作业 SQL Fiddle

Question

查找成员（患者）人数最多的保险公司。

这就是数据库，也是我第一次尝试靠近：

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

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

我如何获得最大计数？

Answer 1

只需按患者数量递减的顺序排列您的结果，并且只记录第一个记录。

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拥有最大数量的成员，并且您想获取所有成员，则需要使用子查询：

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
);

在您在评论中描述的奇怪情况下，无法使用LIMIT，您可以这样做：

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

尝试以下操作：
max(count(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。干得好：

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