What are user defined data types and when you should go for them?
User defined data types let you extend the base SQL Server data types by providing a descriptive name, and format to the database. Take for example, in your database, there is a column called Flight_Num which appears in many tables. In all these tables it should be varchar(8) . In this case you could create a user defined data type called Flight_num_type of varchar(8) and use it across all your tables.