User defined table types (UDT for short) are data types that allows the user to define a table structure. User defined table types supports primary keys, unique constraints and default values.
UDTs have following restrictions -
- can not be used as a column in a table or a field in a structured user-defined types
- a non-clustered index cannot be created in a UDT unless the index is the result of creating a PRIMARY KEY or UNIQUE constraint on the UDT
- UDT definition CANNOT be modified after it is created