mysql group_by , ruby group_by

MySQL教程：Group By用法
GROUP BY (Aggregate) Functions


mysql group_by 弄出來的是只有各一筆的資料, 所以通常會根據某種統計一起用count, max, sum
<br />SELECT store_name, SUM(Sales)　FROM Store_Information GROUP BY store_name　<br />　　#結果:　<br />　　store_name SUM(Sales)<br /><br />　　Los Angeles $1800<br />　　San Diego $250<br />　　Boston $700 <br />

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ruby group_by api
latest_transcripts.group_by(&:day).each do |day, transcripts|<br /> p "#{day} -> #{transcripts.map(&:class).join(', ')}"<br /> end<br /> "2006-03-01 -> Transcript"<br /> "2006-02-28 -> Transcript"<br /> "2006-02-27 -> Transcript, Transcript"<br /> "2006-02-26 -> Transcript, Transcript"<br /> "2006-02-25 -> Transcript"<br /> "2006-02-24 -> Transcript, Transcript"<br /> "2006-02-23 -> Transcript<br />