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$tk_Ids = M("LsnSelected")->where("use_tool=8 AND tk_roomId is null")->select(); (正确)
$tk_Ids = M("LsnSelected")->where("use_tool=8 AND tk_roomId=NULL")->select(); （错误）
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说明：
1、查询字段值为空的语法：where <字段名> is null
2、查询字段值不为空的语法：where <字段名> is not null
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