188.8.131.52 S011, Distinct data types
The specification states the following:
Subclause 11.51, "<user-defined type definition>": When <representation> is <predefined type> <user-defined type definition> ::= CREATE TYPE <user-defined type body> <user-defined type body> ::= <schema-resolved user-defined type name> [ <subtype clause> ] [ AS <representation> ] [ <user-defined type option list> ] [ <method specification list> ] ... <representation> ::= <predefined type> | <member list> Subclause 11.59, "<drop data type statement>": <drop data type statement> ::= DROP TYPE <schema-resolved user-defined type name> <drop behavior>
Microsoft SQL Server 2008 R2 and Microsoft SQL Server 2012 vary as follows:
Transact-SQL does not support this feature. However, Transact-SQL supports user-defined alias types with some equivalent functionality. The user-defined types (UDTs) are not strongly typed, and only the operations that are supported by a UDT's base type are supported.
See [ISO/IEC9075-2:2011] for the definition of "user-defined type (UDT)".