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SQL 的join 区别

2024-07-20 17:19:42   217人阅读

----------------------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

SQL 的join 区别


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