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MySQL: NULLは比較できない

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MySQLに限ったことではないかもしれませんが、NULL値は「IS (NOT) NULL」でしか比較できず...
比較演算子とか、IN句とかどうしても使いたい時は、IFNULL([変換対象], [変換後の値])で一旦何かの文字列に変換してから比較。

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