Question

I wanted to create an outline constraint for an alter key(NOT NULL + UNIQUE), but I think the NOT NULL constraint can’t be placed outline, therefore, I think I have to options:

• Outline constraint: CHECK(attr IS NOT NULL)
• In-line constraint NOT NULL + outline constraint UNIQUE(attr)

Is there any difference between set the in-line constraint NOT NULL to a column and add a constraint CHECK (column IS NOT NULL)?

Thanks in advance

Answer 1

Defining a column as NOT NULL is the preferred approach. That will indicate in the DBA_TAB_COLS, ALL_TAB_COLS, and USER_TAB_COLS data dictionary view, for example, that the column is not NULLABLE. That would be the conventional approach as well so future developers are much more likely to expect that NOT NULL constraints are defined on columns that cannot be NULL.

Just as you could define UNIQUE constraints along with NOT NULL constraints rather than creating PRIMARY KEY constraints, you could define CHECK constraints rather than NOT NULL constraints. Both approaches will work in the same way from a functional standpoint. But the data dictionary views will display those approaches differently so tools that rely on the data dictionary may behave slightly differently. And the conventional approaches are much more likely to be something that future developers will see and expect rather than being surprised by.