部门工资最高的员工

Leecode经典的SQL

Leecode的题目。关于in的应用。感觉很经典，这里列出解题过程。

题目描述

Employee 表包含所有员工信息，每个员工有其对应的 Id, salary 和 department Id。

SQL如下：

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for employee
-- ----------------------------
DROP TABLE IF EXISTS `employee`;
CREATE TABLE `employee`  (
  `Id` int(11) NOT NULL AUTO_INCREMENT,
  `Name` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NULL DEFAULT NULL,
  `Salary` int(255) NULL DEFAULT NULL,
  `DepartmentId` int(255) NULL DEFAULT NULL,
  PRIMARY KEY (`Id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_0900_ai_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of employee
-- ----------------------------
INSERT INTO `employee` VALUES (1, 'Joe', 70000, 1);
INSERT INTO `employee` VALUES (2, 'Jim', 90000, 1);
INSERT INTO `employee` VALUES (3, 'Henry', 80000, 2);
INSERT INTO `employee` VALUES (4, 'Sam', 60000, 2);
INSERT INTO `employee` VALUES (5, 'Max', 90000, 1);

SET FOREIGN_KEY_CHECKS = 1;

Department 表包含公司所有部门的信息。

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for department
-- ----------------------------
DROP TABLE IF EXISTS `department`;
CREATE TABLE `department`  (
  `Id` int(11) NOT NULL AUTO_INCREMENT,
  `Name` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NULL DEFAULT NULL,
  PRIMARY KEY (`Id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_0900_ai_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of department
-- ----------------------------
INSERT INTO `department` VALUES (1, 'IT');
INSERT INTO `department` VALUES (2, 'Sales');

SET FOREIGN_KEY_CHECKS = 1;

解题

方法：使用 JOIN 和 IN 语句
算法

因为 Employee 表包含 Salary 和 DepartmentId 字段，我们可以以此在部门内查询最高工资。

SELECT
    DepartmentId, MAX(Salary)
FROM
    Employee
GROUP BY DepartmentId;

注意：有可能有多个员工同时拥有最高工资，所以最好在这个查询中不包含雇员名字的信息。

DepartmentId	MAX(Salary)
1	90000
2	80000

然后，我们可以把表 Employee 和 Department 连接，再在这张临时表里用 IN 语句查询部门名字和工资的关系。

SELECT
    Department.name AS 'Department',
    Employee.name AS 'Employee',
    Salary
FROM
    Employee
        JOIN
    Department ON Employee.DepartmentId = Department.Id
WHERE
    (Employee.DepartmentId , Salary) IN
    (   SELECT
            DepartmentId, MAX(Salary)
        FROM
            Employee
        GROUP BY DepartmentId
	)
;