MySQL子查询详解(单行子查询、多行子查询与相关子查询) - 主机乐

#回顾：查询员工中工资大于本公司平均工资的员工的last_name,salary和其department_id
SELECT last_name,salary,department_id
FROM employees e1
WHERE salary > (
SELECT AVG(salary)
FROM employees e2
);

#题目：查询员工中工资大于本部门平均工资的员工的last_name,salary和其department_id
SELECT last_name,salary,department_id
FROM employees e1
WHERE salary > (
SELECT AVG(salary)
FROM employees e2
WHERE department_id = e1.`department_id`
);

#方式二,在from中声明子查询
SELECT e.last_name,e.salary,e.department_id
FROM employees e,(SELECT department_id,AVG(salary) avg_sal
FROM employees
GROUP BY department_id) temp
WHERE e.department_id = temp.department_id
AND e.salary > temp.avg_sal;

题目：查询员工的id,salary,按照department_name 排序

SELECT employee_id,salary
FROM employees e1
ORDER BY (
SELECT department_name
FROM departments d
WHERE e1.`department_id` = d.`department_id`
);

题目：若employees表中employee_id与job_history表中employee_id相同的数目不小于2，输出这些相同id的员工的employee_id,last_name和其job_id

#若employees表中employee_id与job_history表中employee_id相同的数目不小于2，输出这些相同
#id的员工的employee_id,last_name和其job_id
SELECT employee_id,last_name,job_id
FROM employees e
WHERE2 <= (SELECT COUNT(*)
FROM job_history j
WHERE e.`employee_id` = j.`employee_id`
);

4.2结论:

在哪里可以写子查询:

在select中,除了GROUP BY 和 LIMIT之外,其他位置都可以声明子查询

4.3EXISTS 与 NOT EXISTS关键字

关联子查询通常也会和 EXISTS操作符一起来使用，用来检查在子查询中是否存在满足条件的行。

如果在子查询中不存在满足条件的行：

条件返回 FALSE
继续在子查询中查找
如果在子查询中存在满足条件的行：
不在子查询中继续查找
条件返回 TRUE
NOT EXISTS关键字表示如果不存在某种条件，则返回TRUE，否则返回FALSE。

题目：查询departments表中，不存在于employees表中的部门的department_id和department_name

#方式一：自连接
SELECT DISTINCT e1.employee_id,e1.last_name,e1.job_id,e1.department_id
FROM employees e1 JOIN employees e2
WHERE e1.`employee_id` = e2.`manager_id`

#方式二：子查询
SELECT DISTINCT manager_id
FROM employees

SELECT employee_id,last_name,job_id,department_id
FROM employees
WHERE employee_id IN (
SELECT DISTINCT manager_id
FROM employees
)

#方式三：exists
SELECT employee_id,last_name,job_id,department_id
FROM employees e1
WHERE EXISTS (
SELECT *
FROM employees e2
WHERE e1.`employee_id`= e2.`manager_id`
)

查询departments表中，不存在于employees表中的部门的department_id和department_name

#方式一：
SELECT d.department_id,d.department_name
FROM employees e RIGHT JOIN departments d
ON e.department_id = d.department_id
WHERE e.department_id IS NULL;

#方式二：
SELECT department_id,department_name
FROM departments d
WHERE NOT EXISTS (
SELECT *
FROM employees e
WHERE d.`department_id` = e.`department_id`
);

5.相关更新

UPDATE table1 alias1
SET column = (SELECT expression
FROM table2 alias2
WHERE alias1.column = alias2.column);

使用相关子查询依据一个表中的数据更新另一个表的数据。

题目：在employees中增加一个department_name字段，数据为员工对应的部门名称

# 1）
ALTER TABLE employees
ADD(department_name VARCHAR2(14));
# 2）
UPDATE employees e
SET department_name = (SELECT department_name
FROM departments d
WHERE e.department_id = d.department_id);

5.相关删除

DELETE FROM table1 alias1
WHERE column operator (SELECT expression
FROM table2 alias2
WHERE alias1.column = alias2.column);

使用相关子查询依据一个表中的数据删除另一个表的数据.

题目：删除表employees中，其与emp_history表皆有的数据

DELETE FROM employees e
WHERE employee_id in
(SELECT employee_id
FROM emp_history
WHERE employee_id = e.employee_id);

问题：谁的工资比Abel的高？

解答：

#方式1：自连接
SELECT e2.last_name,e2.salary
FROM employees e1,employees e2
WHERE e1.last_name = ‘Abel’
AND e1.`salary` < e2.`salary`
#方式2：子查询
SELECT last_name,salary
FROM employees
WHERE salary > (
SELECT salary
FROM employees
WHERE last_name = ‘Abel’
);

问题：以上两种方式有好坏之分吗？

解答：自连接方式好！

题目中可以使用子查询，也可以使用自连接。一般情况建议你使用自连接，因为在许多 DBMS 的处理过程中，对于自连接的处理速度要比子查询快得多。

可以这样理解：子查询实际上是通过未知表进行查询后的条件判断，而自连接是通过已知的自身数据表进行条件判断，因此在大部分 DBMS 中都对自连接处理进行了优化。