DataLink values
A DataLink value is an encapsulated value that contains a logical reference from the database to a file stored outside the database.
The attributes of this encapsulated value are as follows:
- link type
- The currently supported type of link is a URL (Uniform Resource Locator).
- scheme
- For URLs, this is a value such as HTTP or FILE. The value, no matter what case it is entered in, is stored in the database in upper case.
- file server name
- The complete address of the file server. The value, no matter what case it is entered in, is stored in the database in upper case.
- file path
- The identity of the file within the server. The value is case sensitive and therefore it is not converted to upper case when stored in the database.
- access control token
- When appropriate, the access token is embedded within the file path. It is generated dynamically and is not a permanent part of the DataLink value that is stored in the database.
- comment
- Up to 254 bytes of descriptive information. This is intended for application specific uses such as further or alternative identification of the location of the data.
The characters used in a DataLink value are limited to the set defined for a URL. These characters include the uppercase (A through Z) and lower case (a through z) letters, the digits (0 through 9) and a subset of special characters ($, -, _, @, ., &, +, !, *, ", ', (, ), =, ;, /, #, ?, :, space, and comma).
The first four attributes are collectively known as the linkage attributes. It is possible for a DataLink value to have only a comment attribute and no linkage attributes. Such a value may even be stored in a column but, of course, no file will be linked to such a column.
It is important to distinguish between these DataLink references to files and the LOB file reference variables described in References to LOB or XML file reference variables. The similarity is that they both contain a representation of a file. However:
- DataLinks are retained in the database and both the links and the data in the linked files can be considered as a natural extension of data in the database.
- File reference variables exist temporarily and they can be considered as an alternative to a host program buffer.
Built-in scalar functions are provided to build a DataLink value (DLVALUE) and to extract the encapsulated values from a DataLink value (DLCOMMENT, DLLINKTYPE, DLURLCOMPLETE, DLURLPATH, DLURLPATHONLY, DLURLSCHEME, DLURLSERVER).