Ask the new field when the statement ' AAA ' varchar (one) is not NULL, what is the function of NOT NULL?

What is the function of not NULL in the new field when the statement ' AAA ' varchar (one) is not null?
What is the function of not NULL in the new field when the statement ' AAA ' varchar (one) is not null?
is not null possible?
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Indicates that the field cannot be empty (null), that is, when the record is inserted, the field must have a value
is not null possible? Sure, but whether it's based on your business decisions.
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Not NULL is when you insert, this field cannot be null.
Remember, null and empty string are different, empty string is: ' '

Database can declare not null default ' '

In this case, inserting the data, if not declared, is stored by default as an empty string.

There are a number of reasons why not NULL, one of which is that a field in the database is NULL, which represents NULL, but for some language itself and for development reasons, you get it and it becomes a string null.
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field cannot be empty when inserting data, must have value

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