mysql子查询：单行、多行、相关、EXISTS与NOTEXISTS关键字

起步

简介

子查询要包含在括号内
将子查询放在比较条件的右侧
单行操作符对应单行子查询，多行操作符对应多行子查询

分类

单行子查询：查询的结果只有1行
多行子查询：查询的结果有多行
相关(或关联)子查询：子查询中的参数需要用到主查询中的值
不相关(或非关联)子查询

单行

案例1

# 1.查询工资比Abel高用户
# 方式1：需要2步
SELECT salary FROM employees WHERE last_name = 'Abel';

SELECT last_name, salary FROM employees WHERE salary > 11000;

# 方式2：自连接
SELECT e2.last_name, e2.salary
FROM employees e1, employees e2
WHERE e2.`salary` > e1.`salary` #多表的连接条件
AND e1.last_name = 'Abel';

# 方式3：子查询
SELECT last_name,salary
FROM employees
WHERE salary > 
	(SELECT salary
        FROM employees
        WHERE last_name = 'Abel'
        );

称谓的规范

外查询（或主查询）
内查询（或子查询）

单行比较操作符
代码案例

# 查询工资大于149号员工工资的员工的信息
SELECT employee_id, last_name, salary
FROM employees
WHERE salary > (
	SELECT salary
	FROM employees
	WHERE employee_id = 149
	);

# 返回job_id与141号员工相同，salary比143号员工多的员工姓名，job_id和工资
SELECT last_name, job_id, salary
FROM employees
WHERE job_id = (
	SELECT job_id
	FROM employees
	WHERE employee_id = 141
	)
AND salary > (
	SELECT salary
	FROM employees
	WHERE employee_id = 143
	);


# 返回公司工资最少的员工的last_name,job_id和salary
SELECT last_name, job_id, salary
FROM employees
WHERE salary = (
	SELECT MIN(salary)
	FROM employees
	);

# 查询与141号员工的manager_id和department_id相同的其他员工的employee_id，manager_id，department_id
# 方式1：
SELECT employee_id, manager_id, department_id
FROM employees
WHERE manager_id = (
	SELECT manager_id
	FROM employees
	WHERE employee_id = 141
	)
AND department_id = (
	SELECT department_id
	FROM employees
	WHERE employee_id = 141
	)
AND employee_id <> 141;

# 方式2：
SELECT employee_id, manager_id, department_id
FROM employees
WHERE (manager_id,department_id) = (
	SELECT manager_id,department_id
	FROM employees
	WHERE employee_id = 141
	)
AND employee_id <> 141;

# 查询最低工资大于110号部门最低工资的部门id和其最低工资
SELECT department_id, MIN(salary)
FROM employees
WHERE department_id IS NOT NULL
GROUP BY department_id
HAVING MIN(salary) > (
	SELECT MIN(salary)
	FROM employees
	WHERE department_id = 110
	);

# 显式员工的employee_id,last_name和location。其中，若员工department_id与location_id为1800的department_id相同，则location为’Canada’，其余则为’USA’
SELECT employee_id, last_name, CASE department_id WHEN (
	SELECT department_id FROM departments WHERE location_id = 1800) 
THEN 'Canada' ELSE 'USA' END "location" FROM employees;

# 子查询结果为null，则最后结果为null
SELECT last_name, job_id
FROM   employees
WHERE  job_id =
	(SELECT job_id
	FROM   employees
	WHERE  last_name = 'Haas');

多行

简介

也称为集合比较子查询
内查询返回多行
使用多行比较操作符

多行比较操作符
代码案例

# IN:
SELECT employee_id, last_name
FROM employees
WHERE salary IN
	(SELECT MIN(salary)
	FROM employees
	GROUP BY department_id); 
                 
# ANY 或 ALL:
# 返回其它job_id中比job_id为‘IT_PROG’部门任一工资低的员工的员工号、姓名、job_id 以及salary
SELECT employee_id, last_name, job_id, salary
FROM employees
WHERE job_id <> 'IT_PROG'
AND salary < ANY (
	SELECT salary
	FROM employees
	WHERE job_id = 'IT_PROG'
	);

# 返回其它job_id中比job_id为‘IT_PROG’部门所有工资低的员工的员工号、姓名、job_id 以及salary
SELECT employee_id, last_name, job_id, salary
FROM employees
WHERE job_id <> 'IT_PROG'
AND salary < ALL (
	SELECT salary
	FROM employees
	WHERE job_id = 'IT_PROG'
	);
		
# 查询平均工资最低的部门id
# MySQL中聚合函数是不能嵌套使用的。
# 方式1：
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) = (
	SELECT MIN(avg_sal)
	FROM(
		SELECT AVG(salary) avg_sal
		FROM employees
		GROUP BY department_id
		) t_dept_avg_sal
	);

# 方式2：
SELECT department_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) <= ALL(	
	SELECT AVG(salary) avg_sal
	FROM employees
	GROUP BY department_id
	) 

# 子查询结果为null，则最后结果为null
SELECT last_name
FROM employees
WHERE employee_id NOT IN (
	SELECT manager_id
	FROM employees
	);

EXISTS 与 NOT EXISTS关键字

简介

1、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
2、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
3、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
4、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

代码案例

# 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
WHERE 2 <= (
	SELECT COUNT(*)
	FROM job_history j
	WHERE e.`employee_id` = j.`employee_id`
	)

# EXISTS 与 NOT EXISTS关键字
# 查询公司管理者的employee_id，last_name，job_id，department_id信息
# 方式1：自连接
SELECT DISTINCT mgr.employee_id, mgr.last_name, mgr.job_id, mgr.department_id
FROM employees emp JOIN employees mgr
ON emp.manager_id = mgr.employee_id;

# 方式2：子查询
SELECT employee_id, last_name, job_id, department_id
FROM employees
WHERE employee_id IN (
	SELECT DISTINCT manager_id
	FROM employees
	);

#方式3：使用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1：
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2：
SELECT department_id, department_name
FROM departments d
WHERE NOT EXISTS (
	SELECT *
	FROM employees e
	WHERE d.`department_id` = e.`department_id`
	);

posted @ 2022-05-28 13:26 DogLeftover 阅读(88) 评论(0) 收藏举报

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