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
Creating a UDT with a primary key and a column with a default value:
CREATE TYPE MyUniqueNamesType as TABLE ( FirstName varchar(10), LastName varchar(10), CreateDate datetime default GETDATE() PRIMARY KEY (FirstName,LastName) )
creating a UDT with a single int column that is also a primary key
CREATE TYPE dbo.Ids as TABLE ( Id int PRIMARY KEY )
Creating a UDT with a unique constraint:
CREATE TYPE MyUniqueNamesType as TABLE ( FirstName varchar(10), LastName varchar(10), UNIQUE (FirstName,LastName) )
Note: constraints in user defined table types can not be named.
Creating a UDT with multiple columns
CREATE TYPE MyComplexType as TABLE ( Id int, Name varchar(10) )