Mysql 我的sql查询是什么

我的数据库中有两个表

 Table 1. employee
id
name
department_id



 Table 2. department
id
name

现在我有两个问题

1.如何查询以获取其部门的所有员工？ 所以我写了这个查询

SELECT employee.name
     , department.name 
  FROM employee 
  JOIN department 
    ON employee.department_id = department.id

这似乎是正确的，但我无法为下一个问题编写查询

问题是:

---

如果我只想获取员工人数最多的部门？

您的查询应该适用于第一个问题

第二，你可以用这个。子查询将为您提供most员工的部门Id，外部查询将为其提供更多详细信息

select * from department where department_id in
(select limit 1 Employee.department_id from Employee group by department_id 
order by count(Employee.name) desc)

---

这在MySQL中是一个很难处理的问题。这里有一个选择：

SELECT d1.id, d1.name
FROM department d1
INNER JOIN employee e1
    ON d1.id = e1.department_id
GROUP BY d1.id, d1.name
HAVING COUNT(*) = (SELECT COUNT(*) FROM department d2 INNER JOIN employee e2
                       ON d2.id = e2.department_id
                   GROUP BY d2.id ORDER BY COUNT(*) DESC LIMIT 1);

请注意，如果您使用的是具有分析功能支持的数据库，如SQL Server，则问题会变得容易得多：

SELECT id, name
FROM
(
    SELECT d.id, d.name, DENSE_RANK() OVER (ORDER BY COUNT(*) DESC) dr
    FROM department d
    INNER JOIN employee e
        ON d.id = e.department_id
    GROUP BY d.id, d.name
) t
WHERE t.dr = 1;

---

这个问题可以通过多种方式解决：

使用子查询

使用连接

为了保证只有一个部门

SELECT
  *
FROM
  department
WHERE
  id = (SELECT department_id
          FROM employee
      GROUP BY department_id
      ORDER BY COUNT(*) DESC
         LIMIT 1
       )

注意，如果两个部门与联合最大员工数挂钩，这仍然只会选择任意选择的其中一个，每次都可能不同

要处理领带，您可以执行以下操作

SELECT *
  FROM department
 WHERE id IN (SELECT department_id
                FROM employee
            GROUP BY department_id
              HAVING COUNT(*) = (SELECT COUNT(*)
                                   FROM employee
                               GROUP BY department_id
                               ORDER BY COUNT(*) DESC
                                  LIMIT 1
                                )
             )

---

首先检查与列相关的两个类型是否具有相同的名称、相同的数据类型和使用子查询， 按部门id从员工组中选择MAXCOUNTdept\u id中具有COUNTdepartment\u id的员工中选择id所在部门的名称

---

请参见：您需要使用嵌套查询来完成此操作。查看本教程您需要只选择员工最多的部门，还是同时选择该部门的所有员工？@MatBailie是唯一员工最多的部门这是MySQL，不是SQL Server这是一个MySQL问题，不是SQL Server来重复上述评论。对不起，但是MySQL不支持查询的哪一部分？我把这个问题解释为我对SQL的查询，而不是对MySQL的查询。TOP 1 vs LIMIT 1？小提示：你还可以对应用程序代码进行缩进和格式化。同样，使用SQL代码进行缩进和格式化会使写和读一行很长的代码更加方便。@RaymondNijland谢谢。完成！

SELECT
  *
FROM
  department
WHERE
  id = (SELECT department_id
          FROM employee
      GROUP BY department_id
      ORDER BY COUNT(*) DESC
         LIMIT 1
       )

SELECT *
  FROM department
 WHERE id IN (SELECT department_id
                FROM employee
            GROUP BY department_id
              HAVING COUNT(*) = (SELECT COUNT(*)
                                   FROM employee
                               GROUP BY department_id
                               ORDER BY COUNT(*) DESC
                                  LIMIT 1
                                )
             )