concurrency violation [Solve]

Finally, I found my bugs . It is about sql table,
in my sql table, I had already set the default value is '' ( string) and
allow null is false. (e.g deptid)
in my form, deptid may not use, so I didn't put any data in that field.

Now, i found the error is about THAT field. ( If I set 'allow null' is true,
with the default value is '')
I can modify the 'new add' record and save it without concurrency violation.
Also, I find the field (deptid is null but not '')

However, I am wonder why EVEN I set the default value is '' , that field
still got null ???

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