CDEFD

Data Structure Diagrams

A data structure diagram (DSD) is the result of a process of hierarchical decomposition of a complex data area, which is subdivided as far as possible (and reasonable). DSDs are hierarchical tree diagrams depicting "may consist of" relationships between data items if read from top to bottom and following the connecting lines (Diagram 1). The boxes in the diagram may represent

• Intermediate complex data items which are further subdivided in the diagram
• Fully atomized data items, represented by an attribute which can be directly used as a field definition in a database table.
• References to complex data items detailed elsewhere: Instead of listing attributes, a reference to an entire entity may be made (entities may be depicted by a data structure diagram with only one row of data items - attributes - below the header item). Another possibility to make a reference to several data items is to include a part, i.e. a reference to a separate data structure diagram.
• Conditions: The "consists of" relationship may be modified by conditions written in boxes which are marked by an abbreviation on the right hand top of the box defining one of the following conditions:
  "IF": Data items below this box are to be read only if the condition is satisfied
  "EXCL" (= exclusive alternative): Same as "IF", but several such conditions exist which are mutually exclusive
  "LOOP": Data item is repeated as many times as indicated in the condition.

Diagram 1: Data structure diagrams

Attributes are marked by the abbreviation "Attr." on the right hand top of the box. A little arrow pointing to the bottom of a shaded data box indicates that it represents a part, a little arrow pointing to the bottom of an unshaded data box indicates entity. The name of the entity is given in parenthesis in the box.

DSDs may span several entities. Fully resolved DSDs (without references to complex data items) are related to "views" in a relational database system. Loops in the DSD represent a one-to-many relationship if read from the top.