使用子查询的原则
1.一个子查询必须放在圆括号中。
2.将子查询放在比较条件的右边以增加可读性。
子查询不包含 ORDER BY 子句。对一个 SELECT 语句只能用一个 ORDER BY 子句，
并且如果指定了它就必须放在主 SELECT 语句的最后。
ORDER BY 子句可以使用，并且在进行 Top-N 分析时是必须的。
3.在子查询中可以使用两种比较条件：单行运算符和多行运算符。
子查询的类型
单行子查询：从内 SELECT 语句只返回一行的查询
多行子查询：从内 SELECT 语句返回多行的查询
单行子查询
单行子查询是从内查询返回一行的查询。在该子查询类型中用一个单行操作符。幻灯片中列出了单行操作符。
例
显示那些 job ID 与雇员 141 相同的雇员。
SELECT last_name, job_id
FROM employees
WHERE job_id =
(SELECT job_id
FROM employees
WHERE employee_id = 141);
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);
显示 job ID 与雇员 141 相同，并且薪水 高于雇员 143 的那些雇员。
注：外和内查询可以从不同的表中取得数据。
SELECT last_name, job_id, salary
FROM employees
WHERE salary =
(SELECT MIN(salary)
FROM employees);
求所有人谁的工资最小。
SELECT department_id, MIN(salary)
FROM employees
GROUP BY department_id
HAVING MIN(salary) >
(SELECT MIN(salary)
FROM employees
WHERE department_id = 50);
求每个部门的最小工资，但是要高于50号部门的工资。
SELECT employee_id, last_name
FROM employees
WHERE salary =
(SELECT MIN(salary)
FROM employees
GROUP BY department_id);
问题出现在：单行子查询返回了多个查询值；
应改为：
SELECT employee_id, last_name
FROM employees
WHERE salary in
(SELECT MIN(salary)
FROM employees
GROUP BY department_id);
SELECT last_name, job_id
FROM employees
WHERE job_id =
(SELECT job_id
FROM employees
WHERE last_name = 'Haas');
如果子查询返回的是零值，不会对主程序造成影响；
如果子查询返回的是空值，那么会影响主程序的返回值；
SELECT employee_id,
last_name
FROM employees
WHERE employee_id NOT IN
(SELECT manager_id
FROM employees)
SELECT employee_id,
last_name
FROM employees
WHERE employee_id NOT IN
(SELECT manager_id
FROM employees
WHERE manager_id IS NOT NULL)
放在select下的子查询的返回值必须是一个具体值，
from后面也可以加子查询；
having后面也可以加子查询；
order by后面也可以；
多列子查询适应于：成对比较；非成对比较。
SELECT employee_id, manager_id, department_id
FROM employees
WHERE (manager_id, department_id) IN
(SELECT manager_id, department_id
FROM employees
WHERE employee_id IN (178,174))
AND employee_id NOT IN (178,174);
输出：176 149 80
只有要查询的东西和你子查询返回的东西一一对应上了，你的查询才能成功。
如果有一个 对应不上那么你的查询不会成功。
非成对的子查询：
SELECT employee_id, manager_id, department_id
FROM employees
WHERE manager_id IN (SELECT manager_id
FROM employees
WHERE employee_id IN (174,141))
AND department_id IN (SELECT department_id
FROM employees
WHERE employee_id IN (174,141))
AND employee_id NOT IN(174,141);
输出：144 124 50
143 124 50
142 124 50
176 149 80
上面两个程序就是成对子查询和非成对子查询两者之间的区别。
如果我想去显示员工信息，要求：员工的工资高于本部门的平均工资。
SELECT a.last_name,
a.salary,
a.department_id
FROM employees a
WHERE a.salary >
(SELECT AVG(salary)
FROM employees b
WHERE b.department_id = a.department_id);
in line view(内联视图)
SELECT a.last_name, a.salary,
a.department_id, b.salavg
FROM employees a, (SELECT department_id,
AVG(salary) salavg
FROM employees
GROUP BY department_id) b
WHERE a.department_id = b.department_id
AND a.salary > b.salavg;