与 GROUP BY 子句一起使用的 MAX 函数

Question

我有包含员工姓名、薪水和部门的数据库。 我需要查询以获取每个部门中薪水最高的员工。

数据库：

create table test(
    employee_name VARCHAR(255),
    department VARCHAR(255),
    salary INT  
);

数据：

INSERT INTO test(employee_name, department, salary) VALUES 
("John", "DepartmentA", 1500),
("Sarah","DepartmentA", 1600),
("Romel","DepartmentA", 1400),
("Victoria","DepartmentB", 1400),
("Maria",   "DepartmentB", 1600);

我的尝试：

1.1 WHERE MAX(salary) = 工资按部门分组

SELECT employee_name, salary FROM test WHERE MAX(salary) = salary GROUP BY department;
ERROR 1111 (HY000): Invalid use of group function

1.2。当我用硬编码值替换 MAX(salary) 时，它会按我的预期工作：

SELECT employee_name, salary FROM test WHERE 1600 = salary GROUP BY department;
+---------------+--------+
| employee_name | salary |
+---------------+--------+
| Sarah         |   1600 |
| Maria         |   1600 |
+---------------+--------+
2 rows in set (0.00 sec)

1. 有子句的错误答案（单个结果，不是每个部门）：

   SELECTemployee_name,salary FROM test GROUP BY Department HAVING MAX(salary)=salary;

   +---------------+--------+ |员工姓名 |工资 | +---------------+--------+ |玛丽亚 | 1600 | +---------------+--------+ 一组中的 1 行（0.00 秒）

我期望的结果：

Sarah, DepartmentA
Maria, DepartmentB

Answer 1

首先你要得到每个部门的最高薪水：

SELECT department, max(salary) as max_salary
FROM test
GROUP BY department

然后您可以将此子查询加入到测试表中：

SELECT t.*
FROM
  test t INNER JOIN (
    SELECT department, max(salary) as max_salary
    FROM test
    GROUP BY department
  ) d ON t.department=d.department AND t.salary=d.max_salary

Answer 2

试试这个：

SELECT t1.employee_name, t1.department
FROM test t1 
INNER JOIN (SELECT t2.department, MAX(t2.salary) AS salary
            FROM test t2
            GROUP BY t2.department
          ) AS t2 ON t1.department = t2.department AND t1.salary = t2.salary;

Answer 3

您可以将相关子查询编写为：

select employee_name,
department,
salary 
from test T1 where T1.salary = (select max(T2.salary)
                                from test T2
                                where T1.department = T2.department
                                group by T2.department
                                )

demo

...但请注意，它会比不相关的等价物慢得多