内链接
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select * from A inner join B on A.key = B.key;
左连接
第一种
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select * from A left join B on A.key = B.key
第二种
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select * from A left join B on A.key = B.key where B.key is null;
右连接
第一种
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select * from A right join B on A.key = B.key
第二种
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select * from A right join B on A.key = B.key where A.key is null;
全连接
第一种
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select * from A full outer join B on A.key = B.key；
第二种
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