MySQL子查询详解(单行子查询、多行子查询与相关子查询)
0.概念
子查询：一个查询语句嵌套在另一个查询语句内部
1.需求分析与问题解决
1.1提出具体问题：
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)
1.2 子查询的基本使用： 子查询的基本语法结构：
子查询（内查询）在主查询之前一次执行完成。
子查询的结果被主查询（外查询）使用 。
注意事项
- 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
1.3 子查询的分类
角度一:从内查询返回结果的条目数
| 单行子查询 | 多行子查询 |
|---|---|
| 子查询结果只有一个数据 | 子查询数据返回多个 |
角度二:内查询是否被执行多次
| 相关子查询 | 不相关子查询 |
|---|---|
| 查询工资大于本部门平均工资的员工信息 | 查询工资大于本公司平均工资的员工信息 |
2.单行子查询
| 操作符 | 含义 |
|---|---|
| = | equal to |
| > | greater than |
| >= | greater than or equal to |
| < | less than |
| <= | less than or equal to |
| <> | not equal to |
2.1实例：
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)
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
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;
2.2空值问题
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
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
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
SELECT employee_id,last_name,job_id,salary
FROM employees
WHERE salary < ANY(
SELECT salary
FROM employees
WHERE job_id = ‘IT_PROG’
)
AND
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’
)
AND
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
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);
#方式二
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
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
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
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
#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
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
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
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));
# 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
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
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：
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。
总结
到此这篇关于MySQL子查询详解的文章就介绍到这了,更多相关MySQL子查询内容请搜索以前的文章或继续浏览下面的相关文章希望大家以后多多支持！
