我觉得，多表查询，采用匹配的方式要比联接方式要快。即
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select a.*,b.* from a,b where a.id=b.id
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要比
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select a.*,b.* from a inner join b on a.id=b.id
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快。\
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这一点，在外部联接上尤其明显。
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所以，遇到需要用到外部联接，如
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select a.* from a left outer join b on a.id=b.id where a.id=某值\
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我会这样写
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select a.* from a,b where a.id=b.id and a.id=某值
UNION ALL
select a.* from a where a.id=某值 AND NOT EXISTS(SELECT 1 FROM b WHERE id=a.id)
这还不是最优的，还可以这样写
WITH w AS (select * from a WHERE a.id=某值)
select w.* from w,b where w.id=b.id
UNION ALL
select w.* from w where NOT EXISTS(SELECT 1 FROM b WHERE id=w.id)
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究竟是不是这样子呢？存疑。\
