MySQL如何正确查询字段为NULL的字段

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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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