学习目标:
Oracle连接
等值连接:
使用连接在多个表中查询数据。
SELECT table1.column, table2.column
FROM table1, table2
WHERE table1.column1= table2.column2;
from employees e,departments d
where e.department_id = d.department_id
from employees e,departments d,locations l
where e.department_id = d.department_id and
d.location_id = l.location_id
比如:连接三个表,至少须要两个连接条件。
from employees emp,employees manager
where emp.manager_id = manager.employee_id and emp.last_name = 'Chen'
from employees e,job_grades j
where e.salary between j.lowest_sal and j.highest_sal
内连接和外连接:
--左外连接
select e.employee_id,e.last_name,d.department_name
from employees e,departments d
where e.department_id = d.department_id(+)
--右外连接
select e.employee_id,e.last_name,d.department_name
from employees e,departments d
where e.department_id(+) = d.department_id
使用SQL: 1999语法连接:
使用连接从多个表中查询数据:
SELECT table1.column, table2.column
FROM table1
[CROSS JOINtable2] |
[NATURAL JOINtable2] |
[JOINtable2 USING(column_name)] |
[JOINtable2 ON(table1.column_name = table2.column_name)] |
[LEFT|RIGHT|FULL OUTER JOINtable2 ON(table1.column_name =table2.column_name)];
自然连接:
•NATURALJOIN 子句,会以两个表中具有同样名字的列为条件创建等值连接。
•在表中查询满足等值条件的数据。
假设仅仅是列名同样而数据类型不同。则会产生错误。
select e.employee_id,e.last_name,d.department_name
from employees e natural join departments d
使用USING子句创建连接:
JOIN和USING子句常常同一时候使用。
select e.employee_id,e.last_name,d.department_name
from employees e join departments d
using(department_id)
这样的方法有局限性:假设两个表中的列名(一个叫department_id,另外一个叫id)不一样,这种方法就失效了。
使用ON子句创建连接(经常使用):
select e.employee_id,e.last_name,d.department_name
from employees e
join departments d
on e.department_id = d.department_id
这个和等值连接是非常相似的
左外连接
select e.employee_id,e.last_name,d.department_name
from employees e
left outer join departments d
on e.department_id = d.department_id
from employees e
right outer join departments d
on e.department_id = d.department_id
满外连接
select e.employee_id,e.last_name,d.department_name
from employees e
full outer join departments d
on e.department_id = d.department_id