mysql in 子查询 错误 mysql子查询原理

|            40 |  6500.000000 | C           |
|            50 |  3475.555556 | B           |
|            60 |  5760.000000 | B           |
|            70 | 10000.000000 | D           |
|            80 |  8955.882353 | C           |
|            90 | 19333.333333 | E           |
|           100 |  8600.000000 | C           |
|           110 | 10150.000000 | D           |
±--------------±-------------±------------+
12 rows in set (0.00 sec)
where和having后面的子查询
where或having后面，可以使用
1. 标量子查询（单行单列行子查询）
2. 列子查询（单列多行子查询）
3. 行子查询（多行多列）
特点
1. 子查询放在小括号内。
2. 子查询一般放在条件的右侧。
3. 标量子查询，一般搭配着单行操作符使用，多行操作符   >、<、>=、<=、=、<>、!=
4. 列子查询，一般搭配着多行操作符使用
in(not in)：列表中的“任意一个”
any或者some：和子查询返回的“某一个值”比较，比如a>som(10,20,30)，a大于子查询中任意一个即可，a大于子查询中最小值即可，等同于a>min(10,20,30)。
all：和子查询返回的“所有值”比较，比如a>all(10,20,30)，a大于子查询中所有值，换句话说，a大于子查询中最大值即可满足查询条件，等同于a>max(10,20,30);
5. 子查询的执行优先于主查询执行，因为主查询的条件用到了子查询的结果。
mysql中的in、any、some、all
in，any，some，all分别是子查询关键词之一。
in：in常用于where表达式中，其作用是查询某个范围内的数据
any和some一样： 可以与=、>、>=、<、<=、<>结合起来使用，分别表示等于、大于、大于等于、小于、小于等于、不等于其中的任何一个数据。
all：可以与=、>、>=、<、<=、<>结合是来使用，分别表示等于、大于、大于等于、小于、小于等于、不等于其中的其中的所有数据。
下文中会经常用到这些关键字。
标量子查询
一般标量子查询，示例
查询谁的工资比Abel的高？
/①查询abel的工资【改查询是标量子查询】/
SELECT salary
FROM employees
WHERE last_name = ‘Abel’;
/②查询员工信息，满足salary>①的结果/
SELECT *
FROM employees a
WHERE a.salary > (SELECT salary
FROM employees
WHERE last_name = ‘Abel’);
多个标量子查询，示例
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
/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/
/①查询141号员工的job_id/
SELECT job_id
FROM employees
WHERE employee_id = 141;
/②查询143好员工的salary/
SELECT salary
FROM employees
WHERE employee_id = 143;
/③查询员工的姓名、job_id、工资，要求job_id=① and salary>②/
SELECT
a.last_name 姓名,
a.job_id,
a.salary    工资
FROM employees a
WHERE a.job_id = (SELECT job_id
FROM employees
WHERE employee_id = 141)
AND
a.salary > (SELECT salary
FROM employees
WHERE employee_id = 143);
子查询+分组函数，示例
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【having】
/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【having】/
/①查询50号部门的最低工资/
SELECT min(salary)
FROM employees
WHERE department_id = 50;
/②查询每个部门的最低工资/
SELECT
min(salary),
department_id
FROM employees
GROUP BY department_id;
/③在②的基础上筛选，满足min(salary)>①/
SELECT
min(a.salary) minsalary,
department_id
FROM employees a
GROUP BY a.department_id
HAVING min(a.salary) > (SELECT min(salary)
FROM employees
WHERE department_id = 50);
错误的标量子查询，示例
将上面的示例③中子查询语句中的min(salary)改为salary，执行效果如下：
mysql> SELECT
min(a.salary) minsalary,
department_id
FROM employees a
GROUP BY a.department_id
HAVING min(a.salary) > (SELECT salary
FROM employees
WHERE department_id = 500000);
ERROR 1242 (21000): Subquery returns more than 1 row
错误提示：子查询返回的结果超过了1行记录。
说明：上面的子查询只支持最多一列一行记录。
列子查询
列子查询需要搭配多行操作符使用：in(not in)、any/some、all。
为了提升效率，最好去重一下distinct关键字。
示例1
返回location_id是1400或1700的部门中的所有员工姓名
/返回location_id是1400或1700的部门中的所有员工姓名/
/方式1/
/①查询location_id是1400或1700的部门编号/
SELECT DISTINCT department_id
FROM departments
WHERE location_id IN (1400, 1700);
/②查询员工姓名，要求部门是①列表中的某一个/
SELECT a.last_name
FROM employees a
WHERE a.department_id IN (SELECT DISTINCT department_id
FROM departments
WHERE location_id IN (1400, 1700));
/方式2：使用any实现/
SELECT a.last_name
FROM employees a
WHERE a.department_id = ANY (SELECT DISTINCT department_id
FROM departments
WHERE location_id IN (1400, 1700));
/拓展，下面与not in等价/
SELECT a.last_name
FROM employees a
WHERE a.department_id <> ALL (SELECT DISTINCT department_id
FROM departments
WHERE location_id IN (1400, 1700));
示例2
返回其他工种中比job_id为’IT_PROG’工种任意工资低的员工的员工号、姓名、job_id、salary
/返回其他工种中比job_id为’IT_PROG’工种任一工资低的员工的员工号、姓名、job_id、salary/
/①查询job_id为’IT_PROG’部门任-工资/
SELECT DISTINCT salary
FROM employees
WHERE job_id = ‘IT_PROG’;
/②查询员工号、姓名、job_id、salary，slary<①的任意一个/
SELECT
last_name,
employee_id,
job_id,
salary
FROM employees
WHERE salary < ANY (SELECT DISTINCT salary
FROM employees
WHERE job_id = ‘IT_PROG’) AND job_id != ‘IT_PROG’;
/或者/
SELECT
last_name,
employee_id,
job_id,
salary
FROM employees
WHERE salary < (SELECT max(salary)
FROM employees
WHERE job_id = ‘IT_PROG’) AND job_id != ‘IT_PROG’;
示例3
返回其他工种中比job_id为’IT_PROG’部门所有工资低的员工的员工号、姓名、job_id、salary
/返回其他工种中比job_id为’IT_PROG’部门所有工资低的员工的员工号、姓名、job_id、salary/
SELECT
last_name,
employee_id,
job_id,
salary
FROM employees
WHERE salary < ALL (SELECT DISTINCT salary
FROM employees
WHERE job_id = ‘IT_PROG’) AND job_id != ‘IT_PROG’;
/或者/
SELECT
last_name,
employee_id,
job_id,
salary
FROM employees
WHERE salary < (SELECT min(salary)
FROM employees
WHERE job_id = ‘IT_PROG’) AND job_id != ‘IT_PROG’;
行子查询（结果集一行多列）
示例
查询员工编号最小并且工资最高的员工信息，3种方式。
/查询员工编号最小并且工资最高的员工信息/
/①查询最小的员工编号/
SELECT min(employee_id)
FROM employees;
/②查询最高工资/
SELECT max(salary)
FROM employees;
/③方式1：查询员工信息/
SELECT *
FROM employees a
WHERE a.employee_id = (SELECT min(employee_id)
FROM employees)
AND salary = (SELECT max(salary)
FROM employees);
/方式2/
SELECT *
FROM employees a
WHERE (a.employee_id, a.salary) = (SELECT
min(employee_id),
max(salary)
FROM employees);
/方式3/
SELECT *
FROM employees a
WHERE (a.employee_id, a.salary) in (SELECT
min(employee_id),
max(salary)
FROM employees);
方式1比较常见，方式2、3更简洁。
exists后面（也叫做相关子查询）
1. 语法：exists(玩转的查询语句)。
2. exists查询结果：1或0，exists查询的结果用来判断子查询的结果集中是否有值。
3. 一般来说，能用exists的子查询，绝对都能用in代替，所以exists用的少。
4. 和前面的查询不同，这先执行主查询，然后主查询查询的结果，在根据子查询进行过滤，子查询中涉及到主查询中用到的字段，所以叫相关子查询。
示例1
简单示例
mysql> SELECT exists(SELECT employee_id
FROM employees
WHERE salary = 300000) AS ‘exists返回1或者0’;
±---------------------+
| exists返回1或者0     |
±---------------------+
|                    0 |
±---------------------+
1 row in set (0.00 sec)
示例2
查询所有员工的部门名称
/exists入门案例/
SELECT exists(SELECT employee_id
FROM employees
WHERE salary = 300000) AS ‘exists返回1或者0’;
/查询所有员工部门名/
SELECT department_name
FROM departments a
WHERE exists(SELECT 1
FROM employees b
WHERE a.department_id = b.department_id);
/使用in实现/
SELECT department_name
FROM departments a
WHERE a.department_id IN (SELECT department_id
FROM employees);
示例3
查询没有员工的部门
/查询没有员工的部门/
/exists实现/
SELECT *
FROM departments a
WHERE NOT exists(SELECT 1
FROM employees b
WHERE a.department_id = b.department_id AND b.department_id IS NOT NULL);
/in的方式/
SELECT *
FROM departments a
WHERE a.department_id NOT IN (SELECT department_id
FROM employees b
WHERE b.department_id IS NOT NULL);
上面脚本中有b.department_id IS NOT NULL，为什么，有大坑，向下看。
NULL的大坑
示例1
使用in的方式查询没有员工的部门，如下：
SELECT *
FROM departments a
WHERE a.department_id NOT IN (SELECT department_id
FROM employees b);
运行结果：
mysql> SELECT *
-> FROM departments a
-> WHERE a.department_id NOT IN (SELECT department_id