In mysql, how does one judge datetime as null?

In mysql, how does one determine that the time field datetime is null? in mysql, the initial value of the time field datetime is 0000-00-. how does one determine that it is null? if it is null, it is updated to the system time; otherwise, it is not updated, judge a table's field = 0000-00-. if it is waiting, it will be updated. Otherwise, it will not be updated? Why do we have to judge whether it is null? In mysql, how does one judge datetime as null?
In mysql, the initial datetime Value of the time field is 0000-00-00 00:00:00.
How to judge if it is null
Update to system time if it is null
Otherwise, it will not be updated.



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Judge the field of a table = 0000-00-00 00:00:00. if it is waiting, it will be updated; otherwise, it will not be updated.
Is that all right? Why do we have to judge whether it is null?
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It seems that there is no better way to determine whether the strings are equal.
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Yes.
Discussion
It seems that there is no better way to determine whether the strings are equal.