What is rule (database)?

In database, rules are the requirements and restrictions for columns which are stored in the table or value of user-defined data type. Rule is independent database object with separate stored. Rule is independent with the table or user-defined data type. That is to say, when we  delete or modify the table or user-defined data type, it doesn't affect the corresponding rule. Rules and constraints can be used at the same time. CHECK constraints is a more standard method of restrict the column's value when using ALTERTABLE or CREATE TABLE, but CHECK constraints can not act on the user-defined data types directly.

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