File Structure

All Reality files, both dictionaries and data sections, have the same basic structure consisting of a collection of items; the equivalent of records in other databases, but with variable-length format. Each item is structured as a set of numbered attributes; roughly equivalent to fields in records on other databases.

Items have variable-length format and can contain character-oriented or binary-oriented data. In character-oriented data items, attributes can contain multiple values and a value can contain multiple subvalues. Thus Reality allows data to be stored in a three-dimensional structure and variable-length format.

Two-level user file structure

User data is held in a two level file structure comprising data sections which contain the data and a file dictionary which defines the location of one or more data sections, the format of the data in them and the English processing required to generate reports.

Data level descriptors in a file dictionary point to each data section associated with the file, and data definition items, also in the file dictionary, describe the format of the data in the associated data sections, and the English processing to be carried out on named attributes.

Files with no data sections are designated single-level dictionary files. Examples of these are the SYSTEM dictionary and master dictionaries. A single level dictionary file may contain both data items and dictionary items (although this is not recommended), and can be accessed as a dictionary or data file. A single-level dictionary contains an item which points back to itself.

File-specifier

The syntax of a file-specifier allows you to reference a file dictionary or a data section.

Each file dictionary with data sections has a default data section with the same name as the dictionary. Further data sections are given different names according to the rules outlined below.

To access the default data section you only need to specify the file dictionary name. To access a data section other than the default, you must specify the file dictionary name, followed by a comma, then the name of the data section to be accessed.

Syntax

The syntax used to specify a file at TCL is described in general conventions. Note that an alternative syntax, as used on other MultiValue systems, is also supported.

Rules for account names

An account name can be up to 45 characters long. it can contain any characters except spaces, commas, forward slashes, control characters and system delimiters.

The account name must be unique on the database.

For details of how to create accounts, see CREATE-ACCOUNT.

Rules for file and data section names

When creating a new file, bear in mind the following:

• A file or data section name may consist of any sequence of up to 240 non-blank standard ASCII characters, except the following:

  • Control characters.

  • System delimiters.

  • Asterisks (*).

  • Commas (,).

  • Exclamation marks (!).

  • Question marks (?).

  • Semicolons (;).

  • Open parentheses (().

  The first character cannot be a backslash (/); it is recommended you start with an alphanumeric character.

  Note that you can use periods (.) and hyphens (-).

• A file-name must be unique within the account's MD.
• A data section name must be unique within the file's dictionary.

For details of how to create file and data sections, see CREATE-FILE, DIR-VIEW, SQL-VIEW and MAKE-SPECIAL.