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
- 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.
Entity Relation Diagrams
Contents; Entity list;
information. Last updated: April 29, 1996, firstname.lastname@example.org-Berlin.de