SQL 的join 区别

----------------------INNER JOIN--------------------------- 

1. 三表联合查询

select XX,XX  from a , b , c

笛卡尔积，等同于cross join

2. cross join

--列出两边所有组合，也叫笛卡尔集A.Rows * B.Rows

select *

from Sales S cross join Customers C

3. inner join = join

--两边都有的才筛选出来

select *

from Sales S inner join Customers C

on S.Cust_Id = C.Cust_Id

----------------------OUTER JOIN--------------------------- 

4. left join = left outer join

--以左边的表为主表，列出主表所有记录，能匹配就匹配，不匹配的用NULL列出

select * from Customers c left join sales s

on c.cust_id = s.cust_id

5. right join = right outer join

--以右边的表为主表，列出主表所有记录，能匹配就匹配，不匹配的用NULL列出

select * from Customers c right join sales s

on c.cust_id = s.cust_id

6. Full join = full outer join

--两边都列出来，能匹配就匹配，不匹配的用NULL列出

select *

from Sales S full outer join Customers C

on S.Cust_Id = C.Cust_Id

示例======================

SELECT A.id, rule_id, alarm_name, filter_key, A.subtype, B.name
FROM (

A
LEFT JOIN B ON A.subtype = B.id

)
JOIN C ON C.alarm_type = A.type