Question

我使用postgresql作为我的存储引擎。

我有一个员工对经理的自我外键参考。

以下是employee表的外观：

employee_id   firstname  lastname   manager_id
11;           "Chuck";   "Norris";  NULL
1;            "Sergey";  "Brin";    11
3;            "Larry";   "Page";    11
5;            "Bill";    "Gates";   4
4;            "Father";  "Bill";    NULL

现在我想要一个查询来显示拥有大多数员工的员工（manager_id和count）。

对于上表，我希望

    id   max_count
    11        2

此查询有效;

select MAX(y.count) as max_count FROM
(select m.manager_id as id, count(m.manager_id) as count from employee m GROUP BY (id) ) y;

max_count
    2

但这不起作用 - 我只是包含了id列。

select y.id, MAX(y.count) as max_count FROM
(select m.manager_id as id, count(m.manager_id) as count from employee m GROUP BY (id) ) y;

我收到以下错误：

ERROR:  column "y.id" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: select y.id, MAX(y.count) as max_count FROM
               ^

********** Error **********

ERROR: column "y.id" must appear in the GROUP BY clause or be used in an aggregate function
SQL state: 42803

Character: 8

为什么这不起作用？

Answer 1

SELECT列表中的任何非汇总字段都必须包含在GROUP BY中。您可以使用LIMIT或使用窗口/分析函数来获得所需内容：

select y.id, y.count as max_count 
FROM  (select m.manager_id as id
            , count(m.manager_id) as count 
       from employee m 
       GROUP BY id 
      ) y
order by count DESC
limit 1;

正如评论中指出的那样，更为直白：

select m.manager_id as id
     , count(m.manager_id) as count 
from employee m 
GROUP BY id 
order by count(m.manager_id) DESC
limit 1;

如何获取列值以及聚合值（MAX）

1 个答案: