如果部门有超过 20 名员工，则返回部门中最年长员工的信息

Question

不要评判我，我是 SQL 查询的新手。我得到了方案，如下图所示。因此，有 2 个表，第一个员工包含 EmployeeID、FirstName、LastName、DateOfBirth 和 DepartmentID。第二个称为 Department ，包含 DepartmentID 和 DepartmentName 。

我想返回包含超过 20 名员工的每个部门中最年长的员工的 FirstName、LastName 和 DepartmentName。

我的解决方案是以下查询：

SELECT FirstName, LastName, DepartmentName
FROM employees
LEFT JOIN department
ON employees.DepartmentID = department.DepartmentID
    WHERE (employees.DateOfBirth = 
(SELECT MIN(employees.DateOfBirth ) FROM (
SELECT *FROM employees WHERE employees.DepartmentID IN ( 
SELECT employees.DepartmentID FROM employees GROUP BY DepartmentID HAVING COUNT(*) > 20)));

我认为这个逻辑很好，因为内部SELECT 语句将返回每个员工人数超过 20 人的部门的 ID，而外部应该返回最老的员工。

我遇到的问题是，当我尝试执行此查询时，它返回SQL error 每个派生表都必须有自己的别名。 我尝试在每个派生表上放置别名，但结果仍然相同。 请帮我解决这个问题。

另外，如果有人有不同的解决方案，请分享。

谢谢。 草莓要求的加法，Create查询

CREATE TABLE Employees
(
EmployeeID int, 
FirstName varchar(10),
LastName varchar(15),
DateOfBirth date,
DeparmentID int
)

CREATE TABLE Department
(
DepartmentID int, 
DepartmentName varchar(15)
)

Answer 1

由于格式不一致，您的查询难以阅读。所以我会清理如下：

SELECT  FirstName, LastName, DepartmentName
FROM    employees
        LEFT JOIN department
        ON employees.DepartmentID = department.DepartmentID
WHERE   (employees.DateOfBirth = 
        (
        SELECT  MIN(employees.DateOfBirth)
        FROM    (
                SELECT  *
                FROM    employees
                WHERE   employees.DepartmentID IN (
                        --Departments with more than 20 employees
                        SELECT  employees.DepartmentID
                        FROM    employees
                        GROUP BY DepartmentID
                        HAVING COUNT(*) > 20)
                ) -- You need an alias here.
                  -- Also from this point you were missing closing brackets.

您的查询有问题：

• 显然缺少别名和右括号意味着您甚至无法测试您的查询。
• 同样SELECT MIN(employees.DateOfBirth) 只返回一个值。不是每个部门的值。
• 因此，您的总体结果仅包括所有“大”部门中最年长的员工。 （除非每个部门中年龄最大的员工恰好有相同的出生日期。）
• 如果任何员工的出生日期恰好与大部门中最老的相同，它还可以包括来自较小部门的结果。而且那个员工甚至不需要是他们部门中最年长的！
• 使用过多的子查询也会导致效率低下。

CTE（通用表表达式）非常适合简化复杂的查询。但我不知道mysql是否支持它们。所以这个解决方案仍然使用子查询。

SELECT  e.FirstName, e.LastName, d.DepartmentName
FROM    employees e -- I prefer short aliases
        INNER JOIN (
        -- This sub-query returns the earliest birth date within each
        -- big department. This needs to be an aliased query so you
        -- can join to other tables for your desired columns.
        SELECT  DepartmentID, MIN(DateOfBirth) AS MinDOB -- Must alias column
        FROM    employees
        WHERE   DepartmentID IN (
                -- Big departments
                SELECT  DepartmentID
                FROM    employees
                GROUP BY DepartmentID
                HAVING COUNT(*) > 20
                )
        GROUP BY DepartmentID
        ) ddob -- Alias Department Date of Birth
        -- As a result of inner joining to ddob your employees
        -- will be filtered to only those that match the relevant
        -- ones identified in the query.
        ON  e.DepartmentID = ddob.DepartmentID
        AND e.DateOfBirth = ddob.MinDOB
        INNER JOIN Department d
        ON  d.DepartmentID = e.DepartmentID

上述解决方案中需要注意的一点，如果一个部门有 2 名员工因年龄最大而被并列，则两者都会被退回。

这种方法在结构上与您的方法相似，但您也可以从另一个方向解决问题。

• 开始让所有部门中最年长的员工。
• 并且仅在最后根据部门规模过滤结果。

我会留给你去尝试。我怀疑查询会更简单一些。

Answer 2

以下 SQL 脚本对应于您的类图：

CREATE TABLE Departments (
 DepartmentID int AUTO_INCREMENT PRIMARY KEY, 
 DepartmentName varchar(15)
);

CREATE TABLE Employees (
 EmployeeID int AUTO_INCREMENT PRIMARY KEY, 
 FirstName varchar(10),
 LastName varchar(15),
 DateOfBirth date,
 DepartmentID int,
 FOREIGN KEY (DepartmentID) REFERENCES Departments(DepartmentID)
);

根据您的类图，每个员工都有一个部门。所以，这就是使用INNER JOIN 的原因。我认为以下查询可以满足您的要求：

SELECT ee.FirstName, ee.LastName, ee.DateOfBirth, t.DepartmentName 
FROM
( 
     SELECT e.DepartmentID, d.DepartmentName, MIN(e.DateOfBirth) AS DateOfBirth 
     FROM Employees AS e
     INNER JOIN Departments AS d ON e.DepartmentID = d.DepartmentID
     WHERE e.DepartmentID IN (
         SELECT DepartmentID 
         FROM Employees 
         GROUP BY DepartmentID HAVING COUNT(DepartmentID) > 20
     )
     GROUP BY e.DepartmentID
) AS t
INNER JOIN Employees AS ee
WHERE ee.DepartmentID = t.DepartmentID AND ee.DateOfBirth = t.DateOfBirth

输出示例：

FirstName   LastName    DateOfBirth DepartmentName
fisrt14     last14      02/01/2000  SI
fisrt31     last31      12/01/2003  Finance

你提高了它的性能！