回答編集履歴
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追記
answer
CHANGED
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@@ -13,4 +13,21 @@
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13
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and prj.lastupdate=his.updatedt
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left join user as usr
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on usr.id = his.userid
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```
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projectから取得する項目が限定されるなら、いっそ以下の方が高速だと思います。
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```SQL
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select prj.*, usr.username
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from (
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select prj.id, prj.projectname
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, max(his.updatedt) as lastupdate
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from project as prj
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left join history as his
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on prj.id = his.projectid
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group by prj.id, prj.projectname
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) as prj
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left join history as his
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on prj.id = his.projectid
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and prj.lastupdate=his.updatedt
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left join user as usr
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on usr.id = his.userid
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```
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