如何使用子查询获取 MIN/MAX答案

【问题标题】：How to get MIN/MAX with subquery如何使用子查询获取 MIN/MAX
【发布时间】：2015-09-23 17:54:17
【问题描述】：

我有三张这样的桌子

员工

employee_ID(Pk) |部门ID(Fk)

部门

department_ID(Pk) | location_ID(Fk)

地点

location_ID(Pk) |城市

我想要的是员工最少的城市的名称。我尝试了类似下面的 sql ：

SELECT l.city
FROM employees e, departments d, locations l
WHERE e.department_ID = d.department_ID
AND d.location_ID = l.location_ID
GROUP BY l.city
ORDER BY 2 
LIMIT 1

但这不是一个好主意。我希望它在子查询和 MIN 函数中可能是 COUNT 函数。我尝试过但无法弄清楚。有任何想法吗？非常感谢！

【问题讨论】：

标签： mysql sql aggregate-functions

【解决方案1】：

你已经很接近了。试试这个：

select l.city, count(*) as no_of_employees
from locations l
inner join departments d
  on d.location_id = l.location_id
inner join employees e
  on e.department_id = d.department_id
group by l.city
order by no_of_employees asc
limit 1

例子：

create table locations (location_id int, city varchar(20));
insert into locations values (1, 'LA'), (2, 'NY');

create table departments (department_id int, location_id int);
insert into departments values (1, 1), (2, 1), (3, 2), (4, 2);

create table employees (employee_id int, department_id int);
insert into employees values (1, 1), (2, 1), (3, 1), (4, 3), (5, 4);

Result of the query:

| city | no_of_employees |
|------|-----------------|
|   NY |               2 |

SQLFiddle 示例：http://sqlfiddle.com/#!9/75aa1/1

按照评论中的要求使用子查询，您可以这样做 - 但不要这样做！仅在需要时使用子查询。

select * from (
    -- get list of all city and employee count here
    select l.city, count(*) as no_of_employees
    from locations l
    inner join departments d
      on d.location_id = l.location_id
    inner join employees e
      on e.department_id = d.department_id
    group by l.city
) subquery1

-- compare the no of employees with min. employees from the same query
where no_of_employees = (

    -- find minimum number of employees here
    select min(no_of_employees) from (
        -- same query as subquery1
        select l.city, count(*) as no_of_employees
        from locations l
        inner join departments d
          on d.location_id = l.location_id
        inner join employees e
          on e.department_id = d.department_id
        group by l.city
    ) subquery2
)

Result:
| city | no_of_employees |
|------|-----------------|
|   NY |               2 |

SQLFiddle 示例：http://sqlfiddle.com/#!9/75aa1/4

【讨论】：

它有效，谢谢。但它可以通过子查询方式完成吗？我尝试了子查询，但无法正常工作。
尽可能避免子查询。我添加了一个例子
太棒了！！我认为如果多个城市的最低员工人数相同，子查询会有所帮助。还是还有其他的？
@BBCh1 如果您发现此答案有用，请随时将其标记为已接受以结束此问题