1.常用的三种
内连接：select select_list from A inner join B on A.key=B.key
左外连接: select select_list from A left join B on A.key=B.key
右外连接: select select_list from A right join B on A.key=B.key
2.用法升级
左连接：select select_list from A left join B on A.key=B.key where B.key is null
右连接：select select_list from A right join B on A.key=B.key where A.key is null
全连接：
select select_list from A left join B on A.key=B.key
union
select select_list from A right join B on A.key=B.key
全外连接：
select select_list from A left join B on A.key=B.key where B.key is null
union
select select_list from A right join B on A.key=B.key where A.key is null
原文：https://www.cnblogs.com/joeking/p/12542170.html
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