MySQL如何正確查詢字段為NULL的字段

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$tk_Ids = M("LsnSelected")->where("use_tool=8 AND tk_roomId is null")->select();        (正確)

$tk_Ids = M("LsnSelected")->where("use_tool=8 AND tk_roomId=NULL")->select();      （錯誤）

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說明：

1、查詢字段值為空的語法：where <字段名> is null
2、查詢字段值不為空的語法：where <字段名> is not null

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