26. 多表连接查询时, 若两个表有同名的列, 必须使用表的别名对列名进行引用, 否则出错!
 
27. 查询出公司员工的 last_name, department_name, city
 
  
  select last_name, department_name, city
  from departments d, employees e, locations l
  where d.department_id = e.department_id and d.location_id = l.location_id
 
28. 查询出 last_name 为 'Chen' 的 manager 的信息. (员工的 manager_id 是某员工的 employee_id) 
  
  0). 例如: 老张的员工号为: "1001", 我的员工号为: "1002", 
 
            我的 manager_id 为 "1001" --- 我的 manager 是"老张" 
 
  1). 通过两条 sql 查询:
  
      select manager_id
      from employees
      where lower(last_name) = 'chen' --返回的结果为 108
      
      select *
      from employees
      where employee_id = 108
      
  2). 通过一条 sql 查询(自连接):
      
      select m.*
      from employees e, employees m
      where e.manager_id = m.employee_id and e.last_name = 'Chen'   
      
  3). 通过一条 sql 查询(子查询): 
      
      select *
      from employees
      where employee_id = (
                            select manager_id 
                            from employees
                            where last_name = 'Chen'
                          ) 
 
29. 查询每个员工的 last_name 和 GRADE_LEVEL(在 JOB_GRADES 表中). ---- 非等值连接
 
      select last_name, salary, grade_level, lowest_sal, highest_sal
      from employees e, job_grades j
      where e.salary >= j.lowest_sal and e.salary <= j.highest_sal
      
30. 左外连接和右外连接
 
      select last_name, e.department_id, department_name
      from employees e, departments d
      where e.department_id = d.department_id(+)
      
      select last_name, d.department_id, department_name
      from employees e, departments d
      where e.department_id(+) = d.department_id
      
      理解 "(+)" 的位置: 以左外连接为例, 因为左表需要返回更多的记录,
      右表就需要 "加上" 更多的记录, 所以在右表的链接条件上加上 "(+)"
      
      注意: 1). 两边都加上 "(+)" 符号, 会发生语法错误!
            2). 这种语法为 Oracle 所独有, 不能在其它数据库中使用.      
            
31. SQL 99 连接 Employees 表和 Departments 表
      1).
      select *
      from employees join departments
      using(department_id)
      
      缺点: 要求两个表中必须有一样的列名.
      
      2).
      select *
      from employees e join departments d
      on e.department_id = d.department_id
      
      3).多表连接
      select e.last_name, d.department_name, l.city
      from employees e join departments d
      on e.department_id = d.department_id
      join locations l
      on d.location_id = l.location_id           
      
32. SQL 99 的左外连接, 右外连接, 满外连接
      1).
      select last_name, department_name
      from employees e left outer join departments d
      on e.department_id = d.department_id
      
      2).
      select last_name, department_name
      from employees e right join departments d
      on e.department_id = d.department_id
      
      3).
      select last_name, department_name
      from employees e full join departments d
      on e.department_id = d.department_id

1.  显示所有员工的姓名，部门号和部门名称。
a)  select last_name,e.department_id,department_name
b)  from employees e,departments d
c)  where e.department_id = d.department_id(+)
 
方法二：
select last_name,e.department_id,department_name
from employees e left outer join departments d
on e.department_id = d.department_id
2.  查询90号部门员工的job_id和90号部门的location_id
a)  select distinct job_id,location_id
b)  from employees e left outer join departments d
c)  on e.department_id = d.department_id
d)  where d.department_id = 90
3.  选择所有有奖金的员工的
last_name , department_name , location_id , city
select last_name,department_name,d.location_id,city
from employees e join departments d
on e.department_id = d.department_id
join locations l
on d.location_id = l.location_id
where e.commission_pct is not null
4.  选择city在Toronto工作的员工的
last_name , job_id , department_id , department_name 
select last_name , job_id , e.department_id , department_name
from employees e ,departments d,locations l
where e.department_id = d.department_id and l.city = 'Toronto' and d.location_id = l.location_id
5.  选择指定员工的姓名，员工号，以及他的管理者的姓名和员工号，结果类似于下面的格式
employees Emp#  manager Mgr#
kochhar 101 king  100
select e1.last_name "employees",e1.employee_id "Emp#",e2.last_name"Manger",e2.employee_id "Mgr#"
from employees e1,employees e2
where e1.manager_id = e2.employee_id(+)