与 GROUP BY 子句一起使用的 MAX 函数
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【中文标题】与 GROUP BY 子句一起使用的 MAX 函数【英文标题】:MAX function used with GROUP BY clause 【发布时间】:2016-03-24 21:07:16 【问题描述】: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)
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
【问题讨论】:
【参考方案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
【讨论】:
这是正确的答案(我正要发布,只是格式略有不同) 只需要纠正一件小事:GORUP BY -> GROUP BY【参考方案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;
【讨论】:
【参考方案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
【讨论】:
我测试了它,它似乎工作得很好。我投票赞成以上是关于与 GROUP BY 子句一起使用的 MAX 函数的主要内容,如果未能解决你的问题,请参考以下文章
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