與GROUP BY子句一起使用的MAX函數

Question

我擁有名稱，工資和員工部門的數據庫。 我需要一個查詢來獲取每個部門中薪水最高的員工。

數據庫：

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

數據：

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

我嘗試：

1.1 WHERE MAX（工資）=薪水GROUP BY部門

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

1.2。當我硬編碼值替換MAX（工資），它可以作爲我期望：

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) 

錯誤答案與具有條款（而不是每個部門單個結果）：

SELECT employee_name，salary FROM test GROUP BY department HAVING MAX（salary）= salary;

+ --------------- + -------- + | employee_name |薪水| + --------------- + -------- + |瑪麗亞| 1600 | + --------------- + -------- + 1行中集（0.00秒）

我期望的那樣的結果是什麼：

Sarah, DepartmentA 
Maria, DepartmentB 

Answer 1

首先，你必須讓每個部門的最高薪水：

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

，那麼你可以加入這回子查詢測試表：

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

試試這個：

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

您可以編寫相關子查詢爲：

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

...但是請注意，它會比不相關的慢得多相當於