User Defined Types

User-defined types (UDTs) in databases are a feature of SQL that allows users to create their own data types to fulfill specific needs in their databases. UDTs can be used in tables, views, stored procedures, and other database objects, just like any other predefined data types.

A UDT is essentially a data type that a programmer defines, rather than a standard type provided by the SQL language itself. This is comparable to how a programmer might define a new class in an object-oriented programming language. The UDT can encapsulate data and methods that operate on that data.

Here are a couple of examples of how you might use UDTs:

Composite Types: These are user-defined types that can combine several base types. For example, you could define a UDT called "address" that incorporates street, city, state, and zip code fields.
Distinct Types: These are types that are based on a single base type but are considered to be unique. For example, you might define a UDT for a "USPhoneNumber" that is based on the VARCHAR data type, but which you will use only for storing phone numbers in a specific format.

Note that not all database management systems (DBMSs) support UDTs, or they might support them to varying degrees. The SQL standard defines the UDT concept, but implementation can differ across different DBMSs.

In creating UDTs, the objective is to provide a method to extend the database's data type system, encapsulate behavior, improve data integrity, and simplify the database schema.