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I created a table with an explicit null constraint. When I select back from the view "user_constraints" this constraint does not show up. What's going on?
Explicit NULL constraint is a redundant declaration. That is why it is not showing up in user_constraints data dictionary table. Specifying a column as NULL means the column may be kept empty. By not specifying explicitly the null constraint you instruct the DBMS of the same. It is the NOT NULL declaration of a column that instructs the DBMS that the column may not be empty and that will show up in the USER_CONSTRAINTS table for the corresponding column of the table.