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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');多行子查询
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
);相关子查询
子查询中使用主查询中的列
代码案例
# 查询员工中工资大于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
WHERE salary > (
SELECT AVG(salary)
FROM employees
);
# 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
# 方式1：使用相关子查询
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2：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) t_dept_avg_sal # 取别名
WHERE e.department_id = t_dept_avg_sal.department_id
AND e.salary > t_dept_avg_sal.avg_sal
# 查询员工的id,salary,按照department_name排序
SELECT employee_id, salary
FROM employees e
ORDER BY (
SELECT department_name
FROM departments d
WHERE e.`department_id` = d.`department_id`
) ASC;在SELECT中，除了GROUP BY 和 LIMIT之外，其他位置都可以声明子查询
SELECT ....,....,....(存在聚合函数)
FROM ... (LEFT / RIGHT)JOIN ....ON 多表的连接条件
(LEFT / RIGHT)JOIN ... ON ....
WHERE 不包含聚合函数的过滤条件
GROUP BY ...,....
HAVING 包含聚合函数的过滤条件
ORDER BY ....,...(ASC / DESC )
LIMIT ...,....EXISTS 与 NOT EXISTS关键字
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`
);以上就是关于“mysql子查询如何应用”这篇文章的内容，相信大家都有了一定的了解，希望小编分享的内容对大家有帮助，若想了解更多相关的知识内容，请关注亿速云行业资讯频道。
