如何使用单个 SQL 聚合函数查询为同一个聚合函数获取多个结果？答案

【问题标题】：How to get multiple results for the same aggregate function using a single SQL aggregate function query?如何使用单个 SQL 聚合函数查询为同一个聚合函数获取多个结果？
【发布时间】：2021-08-31 22:56:42
【问题描述】：

我是 SQL 新手，我一直在尝试为我的数据库创建视图。当我介绍聚合函数时，我很快就发现了这个问题；

所以在我的数据库中有两个表：用户/员工数据表和组（例如“会计”、“支持”等）。

我想使用查询/视图为该组中的员工返回每个组的整个员工条目以及（该组的）最低/最高工资。

这是表格：

--- 员工数据---

CREATE TABLE `db_java-sql-hookup`.`tbl_employee-data` (
    `ID_Employee` bigint UNIQUE NOT NULL AUTO_INCREMENT,
    `fk_IDGroup` tinyint,
    `EmployeeName` varchar(30) UNIQUE NOT NULL,
    `Email` varchar(255) NOT NULL,
    `Password` varchar(255) NOT NULL,
    `Salary` int,
    PRIMARY KEY (`ID_Employee`),
    FOREIGN KEY (`fk_IDGroup`) REFERENCES `db_java-sql-hookup`.`tbl_groups`(`pk_IDGroup`)
);

INSERT INTO `db_java-sql-hookup`.`tbl_employee-data` (`EmployeeName`, `Email`, `Password`)
VALUES
("TestA", "TestA@web.com", "1234"),
("TestB", "TestB@web.com", "1234"),
("TestC", "TestC@web.com", "abcde"),
("TestD", "TestD@web.com", "0000"),
("TestE", "TestE@web.com", "g8t3");
### Set Groups ###
UPDATE `db_java-sql-hookup`.`tbl_employee-data`
    SET `fk_IDGroup` = 1 WHERE `tbl_employee-data`.`ID_Employee` = 1; 
UPDATE `db_java-sql-hookup`.`tbl_employee-data`
    SET `fk_IDGroup` = 1 WHERE `tbl_employee-data`.`ID_Employee` = 2;
UPDATE `db_java-sql-hookup`.`tbl_employee-data`
    SET `fk_IDGroup` = 1 WHERE `tbl_employee-data`.`ID_Employee` = 3;
UPDATE `db_java-sql-hookup`.`tbl_employee-data`
    SET `fk_IDGroup` = 2 WHERE `tbl_employee-data`.`ID_Employee` = 4;
UPDATE `db_java-sql-hookup`.`tbl_employee-data`
    SET `fk_IDGroup` = 2 WHERE `tbl_employee-data`.`ID_Employee` = 5;
### Set Salaries ###
UPDATE `db_java-sql-hookup`.`tbl_employee-data`
    SET `Salary` = 1000 WHERE `tbl_employee-data`.`ID_Employee` = 1; 
UPDATE `db_java-sql-hookup`.`tbl_employee-data`
    SET `Salary` = 500 WHERE `tbl_employee-data`.`ID_Employee` = 2;
UPDATE `db_java-sql-hookup`.`tbl_employee-data`
    SET `Salary` = 2000 WHERE `tbl_employee-data`.`ID_Employee` = 3;
UPDATE `db_java-sql-hookup`.`tbl_employee-data`
    SET `Salary` = 750 WHERE `tbl_employee-data`.`ID_Employee` = 4;
UPDATE `db_java-sql-hookup`.`tbl_employee-data`
    SET `Salary` = 300 WHERE `tbl_employee-data`.`ID_Employee` = 5;

---分组数据---

CREATE TABLE `db_java-sql-hookup`.`tbl_groups` (
    `pk_IDGroup` tinyint UNIQUE NOT NULL AUTO_INCREMENT,
    `GroupName` varchar(40) NOT NULL,
    PRIMARY KEY (`pk_IDGroup`)
);

INSERT INTO `db_java-sql-hookup`.`tbl_groups` (`GroupName`)
VALUES 
    ("Accounting"),
    ("Support"),
    ("Development"),
    ("Test");

之前，我处理过这些视图（我想将它们结合起来）：

### MaxSalaryEmp ###    
DROP VIEW IF EXISTS `db_java-sql-hookup`.`view_MaxSalaryEmployee`;
CREATE VIEW `db_java-sql-hookup`.`view_MaxSalaryEmployee` AS    
    SELECT `ID_Employee`, `EmployeeName`, `Salary`
    FROM `db_java-sql-hookup`.`tbl_employee-data`
    WHERE `Salary` = 
        (SELECT MAX(`Salary`) FROM `db_java-sql-hookup`.`tbl_employee-data`);

### Avg,Min,Max Group Salary ###
DROP VIEW IF EXISTS `db_java-sql-hookup`.`view_CombinedGroupSalary`;
CREATE VIEW `db_java-sql-hookup`.`view_CombinedGroupSalary` AS   
    SELECT `GroupName`, 
        AVG(`Salary`) AS `AvgSalary`,
        MIN(`Salary`) AS `MinSalary`,
        MAX(`Salary`) AS `MaxSalary`
    FROM `db_java-sql-hookup`.`tbl_groups` AS grp
    LEFT JOIN `db_java-sql-hookup`.`tbl_employee-data` AS emp
    ON grp.`pk_IDGroup` = emp.`fk_IDGroup`
    GROUP BY `GroupName`
    ORDER BY `GroupName`;

我尝试过这样的事情：

SELECT `GroupName`, `ID_Employee`, `EmployeeName`,  `Salary`,
    MAX(`Salary`) AS `MaxSalary`
FROM `db_java-sql-hookup`.`tbl_groups` AS grp
LEFT JOIN `db_java-sql-hookup`.`tbl_employee-data` AS emp
ON grp.`pk_IDGroup` = emp.`fk_IDGroup`
GROUP BY `GroupName`
ORDER BY `GroupName`;

我希望最终结果如下所示： https://i.stack.imgur.com/xsmLT.png

（除了，它应该派出合适的员工而不是这里发生的任何事情）

提前谢谢你！

【问题讨论】：

标签： sql aggregate-functions

【解决方案1】：

我不清楚你想要完成什么。您只想要每个组的顶级员工还是想要按组列出所有员工并显示每个组的最高薪水？

如果您希望每组一个条目，请使用您的组合工资视图，如下所示：

SELECT grp.`GroupName`, emp.`ID_Employee`, emp.`EmployeeName`,  emp.`Salary`,
   grp.`MaxSalary` 
FROM  `db_java-sql-hookup`.`view_CombinedGroupSalary` grp
LEFT JOIN `db_java-sql-hookup`.`tbl_employee-data` AS emp
ON grp.`pk_IDGroup` = emp.`fk_IDGroup` and grp.`MaxSalary` = emp.`Salary`
ORDER BY `GroupName`;

如果您需要列出所有员工，请执行以下操作：

SELECT grp.`GroupName`, emp.`ID_Employee`, emp.`EmployeeName`,  emp.`Salary`,
   cs.`MaxSalary`
FROM `db_java-sql-hookup`.`tbl_groups grp`
INNER JOIN `db_java-sql-hookup`.`view_CombinedGroupSalary cs 
on grp.`pk_IDGroup` = cs.`pk_IDGroup`
LEFT JOIN `db_java-sql-hookup`.`tbl_employee_data` AS emp
ON grp.`pk_IDGroup` = emp.`fk_IDGroup`
ORDER BY grp.`GroupName`, emp.`EmployeeName`;

要使这些查询起作用，您需要在组合工资视图中包含 pk_IDGroup 列：

DROP VIEW IF EXISTS `db_java-sql-hookup`.`view_CombinedGroupSalary`;
CREATE VIEW `db_java-sql-hookup`.`view_CombinedGroupSalary` AS   
    SELECT `pk_IDGroup`,
        `GroupName`, 
        AVG(`Salary`) AS `AvgSalary`,
        MIN(`Salary`) AS `MinSalary`,
        MAX(`Salary`) AS `MaxSalary`
    FROM `db_java-sql-hookup`.`tbl_groups` AS grp
    LEFT JOIN `db_java-sql-hookup`.`tbl_employee-data` AS emp
    ON grp.`pk_IDGroup` = emp.`fk_IDGroup`
    GROUP BY `pk_IDGroup`,`GroupName`
    ORDER BY `GroupName`;

【讨论】：

非常感谢！第一个查询实际上正是我想要的。我不知道我也可以加入视图。

【解决方案2】：

对于这个问题，我认为你不需要使用 group by，而是需要使用“user-defined-variables”：

这个link 可以告诉你如何在 mysql 中按分区获取最大值（在 mssql 中它的工作方式不同）。

这是我需要删除“db_java-sql-hookup”的 sql 小提琴中的问题 (sqlfiddle) 的解决方案。从表格中，您可以使用以下代码：

select `GroupName`, `ID_Employee`, `EmployeeName`,  `Salary`,
       @max:=IF(@custtype!=`GroupName`,`Salary`,@max),
       @custtype:=`GroupName`
FROM (
  select `GroupName`, `ID_Employee`, `EmployeeName`,  `Salary`
  FROM `db_java-sql-hookup`.`tbl_groups` AS grp
  LEFT JOIN `db_java-sql-hookup`.`tbl_employee-data` AS emp
  ON grp.`pk_IDGroup` = emp.`fk_IDGroup`
  order by `GroupName`, `Salary` desc
) a, (select @max:=0, @custtype:='') t

如果您需要有关其工作原理的更多详细信息，请告诉我，我会添加它。

【讨论】：