Rules
You can use rules as another method to enforce domain integrity. Rules are similar to CHECK constraints but have some limitations. The biggest advantage when using a rule is that one rule can be bound to multiple columns or user-defined data types. This capability can be useful for columns that contain the same type of data and are found in multiple tables in a database. The syntax for creating a rule is as follows:
CREATE RULE [ schema_name . ] rule_nameAS condition_expression[ ; ]
condition_expression can include any statement that can be placed in a WHERE clause. It includes one variable that is preceded with the @ symbol. This variable contains the value of the bound column that is supplied with the
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