Specifying NOT NULL

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You can optionally specify NOT NULL to force the user to enter a value. If you do not specify NOT NULL, then NULL values are allowed for any column that references this domain. NOT NULL specified on the domain level cannot be overridden by a local column definition.

Important: If you have already specified NULL as a default value, be sure not to create contradictory constraints by also assigning NOT NULL to the domain, as in the following example:
CREATE DOMAIN DOM1 INTEGER DEFAULT NULL, NOT NULL;

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