Ask a simple question about SQL

sql Freshmen , Because I can't find the reason, I really can't figure it out. So I come to ask you for help , I really can't figure it out. I'm so bored

Why is it written here WHERE building Just take it out null value , I didn't even write WHERE building is not null

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Take the answer ：

WHERE The core of the following expression is actually the Boolean value .
building This field is implicitly converted to a Boolean value here , Boolean values have three values , One is true, One is false, The other one is null value ,where as long as true Of , That is, the data with actual value in this field , There is data filtering .

I hope I can help you :)

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