mysql 7 种 join
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一、
select * from A inner join B on A.key = B.key
二、
select * from A left join B on A.key = B.key
三、
select * from A right join B on A.key = B.key
四、
select * from A left join B on A.key = B.key where b.key is null
五、
select * from A right join B on A.key = B.key where A.key is null
六、
select * from A left join B on A.key = B.key union select * from A right join B on A.key = B.key
七、
select * from A left join B on A.key = B.key where B.key is null union select * from A right join B on A.key = B.key where A.key is null
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