所谓子查询是指在一个查询中嵌套了其他的若干查询,即在一个SELECT查询语句的WHERE或FROM子句中包含另一个SELECT查询语句,下面这篇文章主要给大家介绍了关于MySQL单行子查询、多行子查询与相关子查询的相关资料,需要的朋友可以参考下
# 法一:效率低
SELECT last_name,salary
FROM employees
WHERE last_name = 'ABEL'
SELECT last_name,salary
FROM employees
WHERE salary > 11000;
#法二：自连接
SELECT e2.last_name,e2.salary
FROM employees e1,employees e2
WHERE e2.`salary` > e1.`salary`
AND e1.`last_name` = 'Abel';
#法三：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(主查询),内查询(子查询)
# 查询工资大于149号员工工资的信息
SELECT salary,last_name,employee_id
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)
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
SELECT employee_id,manager_id,department_id
FROM employees
WHERE
manager_id = (SELECT manager_id
FROM employees
WHERE employee_id = 141)
department_id =(SELECT department_id
FROM employees
WHERE employee_id = 141)
employee_id <> 141;
#方式二：成对查询
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50号部门最低工资的部门id和其最低工资
SELECT MIN(salary),department_id,salary
FROM employees
GROUP BY department_id
HAVING MIN(salary) > (
SELECT MIN(salary)
FROM employees
WHERE department_id = 50
ORDER BY MIN(salary) DESC;
# 题目：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;
SELECT last_name
FROM employees
WHERE employee_id NOT IN (
SELECT manager_id
FROM employees
内查询的结果是NULL空值,不会报错,但是也不会显示数据
2.3非法使用子查询
SELECT employee_id, last_name
FROM employees
WHERE salary =
(SELECT MIN(salary)
FROM employees
GROUP BY department_id);
这里内查询返回的结果是一列数据,不能使用等于号连接,必须使用in
3.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
3.1多行比较操作符
| 操作符 | 含义 |
|---|
| IN | 等于列表中的任意一个 |
| ANY | 需要和单行比较操作符一起使用，和子查询返回的某一个值比较 |
| ALL | 需要和单行比较操作符一起使用，和子查询返回的所有值比较 |
| SOME | 实际上是ANY的别名，作用相同，一般常使用ANY |
3.2代码实例
SELECT employee_id, last_name
FROM employees
WHERE salary in
(SELECT MIN(salary)
FROM employees
GROUP BY department_id);
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
#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 salary < ANY(
SELECT salary
FROM employees
WHERE job_id = 'IT_PROG'
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 salary < ALL(
SELECT salary
FROM employees
WHERE job_id = 'IT_PROG'
job_id <> 'IT_PROG';
查询平均工资最低的部门id
相当于创建了一张临时的表
# 聚合函数不能嵌套，单行函数才能嵌套使用
SELECT employee_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);
SELECT employee_id
FROM employees
GROUP BY department_id
HAVING AVG(salary) <= ALL(
SELECT AVG(salary) avg_sal
FROM employees
GROUP BY department_id
3.3空值问题
SELECT last_name
FROM employees
WHERE employee_id NOT IN (
SELECT manager_id
FROM employees
#where manager_id is not null
4.相关子查询
如果子查询的执行依赖于外部查询，通常情况下都是因为子查询中的表用到了外部的表，并进行了条件关联，因此每执行一次外部查询，子查询都要重新计算一次，这样的子查询就称之为 关联子查询 .相关子查询按照一行接一行的顺序执行，主查询的每一行都执行一次子查询。
4.1代码实例
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
#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
WHERE 2 <= (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
ALTER TABLE employees
ADD(department_name VARCHAR2(14));
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？
