我能知道相关子查询是什么意思吗？

Question

我有一个名为 sql_hr 的数据库，其中有一个 table employees。

下面是一个 .sql 文件的内容，该文件创建 sql_hr 数据库，其中包含 table employees：

DROP DATABASE IF EXISTS `sql_hr`;
CREATE DATABASE `sql_hr`;
USE `sql_hr`;


CREATE TABLE `employees` (
  `employee_id` int(11) NOT NULL,
  `first_name` varchar(50) NOT NULL,
  `last_name` varchar(50) NOT NULL,
  `job_title` varchar(50) NOT NULL,
  `salary` int(11) NOT NULL,
  `reports_to` int(11) DEFAULT NULL,
  `office_id` int(11) NOT NULL,
  PRIMARY KEY (`employee_id`),
  KEY `fk_employees_offices_idx` (`office_id`),
  KEY `fk_employees_employees_idx` (`reports_to`),
  CONSTRAINT `fk_employees_managers` FOREIGN KEY (`reports_to`) REFERENCES `employees` (`employee_id`),
  CONSTRAINT `fk_employees_offices` FOREIGN KEY (`office_id`) REFERENCES `offices` (`office_id`) ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;
INSERT INTO `employees` VALUES (37270,'Yovonnda','Magrannell','Executive Secretary',63996,NULL,10);
INSERT INTO `employees` VALUES (33391,'D\'arcy','Nortunen','Account Executive',62871,37270,1);
INSERT INTO `employees` VALUES (37851,'Sayer','Matterson','Statistician III',98926,37270,1);
INSERT INTO `employees` VALUES (40448,'Mindy','Crissil','Staff Scientist',94860,37270,1);
INSERT INTO `employees` VALUES (56274,'Keriann','Alloisi','VP Marketing',110150,37270,1);
INSERT INTO `employees` VALUES (63196,'Alaster','Scutchin','Assistant Professor',32179,37270,2);
INSERT INTO `employees` VALUES (67009,'North','de Clerc','VP Product Management',114257,37270,2);
INSERT INTO `employees` VALUES (67370,'Elladine','Rising','Social Worker',96767,37270,2);
INSERT INTO `employees` VALUES (68249,'Nisse','Voysey','Financial Advisor',52832,37270,2);
INSERT INTO `employees` VALUES (72540,'Guthrey','Iacopetti','Office Assistant I',117690,37270,3);
INSERT INTO `employees` VALUES (72913,'Kass','Hefferan','Computer Systems Analyst IV',96401,37270,3);
INSERT INTO `employees` VALUES (75900,'Virge','Goodrum','Information Systems Manager',54578,37270,3);
INSERT INTO `employees` VALUES (76196,'Mirilla','Janowski','Cost Accountant',119241,37270,3);
INSERT INTO `employees` VALUES (80529,'Lynde','Aronson','Junior Executive',77182,37270,4);
INSERT INTO `employees` VALUES (80679,'Mildrid','Sokale','Geologist II',67987,37270,4);
INSERT INTO `employees` VALUES (84791,'Hazel','Tarbert','General Manager',93760,37270,4);
INSERT INTO `employees` VALUES (95213,'Cole','Kesterton','Pharmacist',86119,37270,4);
INSERT INTO `employees` VALUES (96513,'Theresa','Binney','Food Chemist',47354,37270,5);
INSERT INTO `employees` VALUES (98374,'Estrellita','Daleman','Staff Accountant IV',70187,37270,5);
INSERT INTO `employees` VALUES (115357,'Ivy','Fearey','Structural Engineer',92710,37270,5);

好的，现在显示 table employees:

USE sql_hr;
SELECT * FROM employees;

我想向 return 仅针对具有相同 office_id 的每组员工中的一些客户编写查询，基于这些客户的标准应该比所有员工的平均工资多 salary具有特定 office_id.

的客户现在，我想分享两个查询 (1) 和 (2)，其中输出已在查询后显示：

(1):

SELECT
    employee_id,
    first_name,
    salary
FROM employees e

WHERE salary> (
    SELECT AVG(salary)
    FROM employees
    WHERE office_id= e.office_id);

(2):

SELECT
    employee_id,
    first_name,
    salary
FROM employees

WHERE salary> ALL(
    SELECT AVG(salary)
    FROM employees
    GROUP BY office_id);

我认为以上两个查询的意思相同。但事实证明，查询（1）根据问题是正确的，但不是（2）。为什么查询 (2) 与查询 (1) 不同？

Answer 1

第一个查询 returns 工资高于其所属 office_id 平均工资的员工。
这是此条件的作用：

WHERE salary> (
    SELECT AVG(salary)
    FROM employees
    WHERE office_id= e.office_id);

第二个查询returns薪水大于所有平均薪水的员工意思：薪水大于最大平均的员工office_ids.
这是条件：

WHERE salary> ALL(
    SELECT AVG(salary)
    FROM employees
    GROUP BY office_id);

子查询：

SELECT AVG(salary)
FROM employees
GROUP BY office_id

returns 所有 office_id 的所有平均值。
因此，当您希望薪水大于所有薪水时，您的意思是 大于所有薪水的最大值 .

我能知道相关子查询是什么意思吗？

Can I know what correlated sub-queries mean?

mysql

sql

database

group-by

correlated-subquery