Overview of OLE DB Data Types

Data is defined as and stored by many types, so it is required that both the provider and consumer be able to understand the type. Presented here is the topic of determining data type in OLE DB by the use of type indicators when creating tables and getting and setting data, handling string data, fixed- and variable-length data, and converting data. Type indicators in an ANSI SQL Provider, a Microsoft Access provider, and a Microsoft service component are discussed in detail. Determining the type of data from a string (collectively referring to ANSI, Unicode, and BSTR), and provider support for data type conversions are also presented.

OLE DB uses standard OLE and Windows data types. To describe a data type, a type indicator is used. This is a variable of the enumerated type DBTYPE that is passed as metadata. Although it is easy to confuse the two, type indicators are not data types. For example, DBTYPE_GUID is an integer with a value of 72, not a GUID. It is used to indicate that a piece of memory contains a GUID. Type indicators are used in many places, including the following:

• Metadata. Type indicators are used to specify the data type of a column or parameter.
  
  
• DBBINDING structures. The wType element of the DBBINDING structure describes the type of the bound data value. When getting data, this is the type to which the provider converts the data before returning it to the consumer. When setting data, this is the type from which the provider converts the data after retrieving it from the consumer.
  
  
• VARIANT structures. The vt element, a short integer, of the VARIANT structure indicates the type of the data that is stored in the structure. The remainder of the structure is 14 bytes, which are structured according to the vt tag. The DBTYPE enumerated type is a superset of the VARENUM enumerated type, so some DBTYPE values cannot be used in VARIANT structures.

For a complete list of type indicators, see "Type Indicators" in Appendix A.