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集計時、グループ内の種類のカウントをするSQL

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SQLでグループ内の種類をカウントしたい場合は以下のようにすると、取得できる。

COUNTでNULLをカウントされない仕様を利用しているから、データベースによっては利用できないかも。
(Oracleだと使える。)

社員テーブルの部内の課の種類の数をカウントしている。

sample.sql

SELECT 部
     , COUNT(DISTINCT CASE WHEN 課 = 'ABC'
                           THEN NULL
                           ELSE 課 END) 課件数
     , COUNT(社員ID) 社員件数
  FROM 社員
 GROUP BY 部
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