A distinct type is a user-defined data type that shares its internal representation with an existing type (its "source" type), but is considered to be a separate and incompatible type for most operations. For example, one might want to define a picture type, a text type, and an audio type, all of which have quite different semantics, but which use the built-in data type BLOB for their internal representation.
The following example illustrates the creation of a distinct type named AUDIO:
CREATE DISTINCT TYPE AUDIO AS BLOB (1M)
Although AUDIO has the same representation as the built-in data type BLOB, it is considered to be a separate type that is not comparable to a BLOB or to any other type. This allows the creation of functions written specifically for AUDIO and assures that these functions will not be applied to any other type (pictures, text, etc.).
Distinct types are identified by qualified identifiers. If the schema name is not used to qualify the distinct type name when used in other than the CREATE DISTINCT TYPE, DROP DISTINCT TYPE, or COMMENT ON DISTINCT TYPE statements, the SQL path is searched in sequence for the first schema with a distinct type that matches. The SQL path is described in CURRENT PATH.
Distinct types support strong typing by ensuring that only those functions and operators explicitly defined on a distinct type can be applied to its instances. For this reason, a distinct type does not automatically acquire the functions and operators of its source type, since these may not be meaningful. (For example, the LENGTH function of the AUDIO type might return the length of its object in seconds rather than in bytes.)
Distinct types sourced on LONG VARCHAR, LONG VARGRAPHIC, LOB types, or DATALINK are subject to the same restrictions as their source type.
However, certain functions and operators of the source type can be explicitly specified to apply to the distinct type by defining user-defined functions that are sourced on functions defined on the source type of the distinct type (see User-defined Type Comparisons for examples). The comparison operators are automatically generated for user-defined distinct types, except those using LONG VARCHAR, LONG VARGRAPHIC, BLOB, CLOB, DBCLOB, or DATALINK as the source type. In addition, functions are generated to support casting from the source type to the distinct type and from the distinct type to the source type.