select e.last_name,d.department_name
from employees e,departments d
where last_name='grant'and e.department_id = d.department_id
等價於：
select e.last_name,d.department_name
from employees e join departments d on e.department_id=d.department_id
where last_name='grant'
lesson 7
natural joins(等值連線)
-natural join 要求連線的兩列資料型別相同(等值連線)；不用指定列，自動匹配名字內容相同的類；
-using clause using 加的列的名字不能在其他子句中限定條件；
-on clause on指定條件;(相當於where);
lesson 8
非等值連線
select e.ename,s.grade
from emp e,salgrade s
where e.sal between s.losal and s.hisal and e.ename='smith'
outer joins
left outer join(只顯示左表中不匹配的記錄)
例: select e.last_name,e.department_id,d.department_id
from employees e left [outer] join departments d
on (e.department_id = d.department_id)
(把e中不匹配的顯示出來)
(oracle語法)
select e.last_name,d.department_id,d.department_id
from employees e left outer join departments d
where e.department_id = d.department_id(+)
right outer join(只顯示右表中不匹配的記錄)
select e.last_name,d.department_id,d.department_id
from employees e right [outer] join departments d
on (e.department_id = d.department_id)
(把d中不匹配的顯示出來)
(oracle語法)
select e.last_name,e.department_id,d.department_id
from employees e left outer join departments d
where e.department_id(+) = d.department_id
full outer join(左右兩個表都顯示)
select e.last_name,d.department_id,d.department_id
from employees e full [outer] join departments d
on (e.department_id = d.department_id)
cartesian product(笛卡爾集)做測試資料用；
cross join(當沒有連線條件時，或乙個無效的連線條件)
select last_name,department_name
from employees
cross join departments(輸出結果為第乙個表的行數*第二個表的行數)
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