查询简化Oracle Northwind

Question

任务：

查找最大客户所在的国家/地区。

查询

SELECT country,
       count(*)
FROM customers
GROUP BY country
HAVING count(*) =
  (SELECT max(max_sal)
   FROM
     (SELECT count(*) max_sal
      FROM customers
      GROUP BY country)) ;

结果：

结果是正确的，但我认为编写查询很难。

问题：有没有简单的方法可以重写此查询。

Answer 1

我可能会遗漏一些东西，但它可以像这样简单：

SELECT *
  FROM (  SELECT country, COUNT (*) max_sal
            FROM customers
        GROUP BY country
        ORDER BY COUNT (*) DESC)
 WHERE ROWNUM <= 1;

Answer 2

您可以使用WITH子句：

WITH
  c AS ( 
    SELECT country, Count(1) n
    FROM customers
    GROUP BY country)
SELECT country, n 
FROM c
WHERE n = (SELECT Max(n) FROM c)

Answer 3

你可以使用解析函数over（）你得到每行结果的max（avg，min等）结果然后在compare count（1）和max（count（1））

这里是例子：

SELECT country, cnt, max_cnt
  FROM (SELECT country, COUNT(1) AS cnt, MAX(COUNT(1)) over() max_cnt
           FROM customers
          GROUP BY country)
 WHERE cnt = max_cnt

Answer 4

SELECT Country, MAX(N) FROM (
    SELECT Country, COUNT(*) as N FROM CUSTOMERS GROUP BY Country
);