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mysqlnotin、leftjoin、ISNULL、NOTEXISTS效率问题记录_MySQL:bitsCN.com NOT IN、JOIN、IS NULL、NOT EXISTS效率对比 语句一:select count(*) from A where A.a not in (select a from B) 语句二:select count(*) from A left join B on A.a = B.a where B.a is
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