Questions about the NOT null field for MySQL

About the NOT null field problem with MySQL
Today, I have a very painful problem, about MySQL database, a set NOT null integer field, the default value is 2, when I insert a null value to the field, why is the system default auto-generated integer default value 0, not my custom default value 2, very tangled, if I do not do any action on the field, it will automatically populate the custom default values, why this, ask Gao, thank you ....

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A null value is also a value, that is, the default value is not used. If you leave blank, the default value will be used.
Well, that's a good understanding.
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Null means nothing at all
Since the value Type field is set to NOT NULL
So nothing in math is 0?
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If you are self-defined default value, do not set to not NULL, when inserted directly without this field, is the default value
Cannot be empty meaning you have to insert a value, even if it is a null value, the default value is useless.
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Explore

WorkID ' int (ten) unsigned not NULL default ' 3 '

When I insert a null value into this field, why does it become 0?

Reference:

If you are self-defined default value, do not set to not NULL, when inserted directly without this field, is the default value
Cannot be empty meaning you have to insert a value, even if it is a null value, the default value is useless.

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