My first Publication Agile-Data-Warehouse-Design-eBook | Page 26

How to Model a Data Warehouse
5
The Case Against Entity-Relationship Modeling
Entity-Relationship (ER) modeling is the standard approach to data modeling for
OLTP database design. It classifies all data as one of three things: an entity, a
relationship, or an attribute. Figure 1-1 shows an example entity-level ER diagram
(ERD). Entities are shown as boxes and relationships as lines linking the boxes.
The cardinality of each relationship — the number of possible matching values on
either side of the relationship — is shown using crow’s feet for many, | for one, and
O for zero (also knowO as optionality).
ER modeling is
used to design
OLTP databases
Figure 1-1
Entity-Relationship
diagram (ERD)
Within a relational database, entities are implemented as tables and their attributes
as columns. Relationships are implemented either as columns within existing
tables or as additional tables depending on their cardinality. One-to-one (1:1) and
many-to-one (M:1) relationships are implemented as columns, whereas many-to-
many (M:M) relationships are implemented using additional tables, creating
additional M:1 relationships. Entities become
ER modeling is associated with normalization in general, and third normal form
(3NF) in particular. ER modeling and normalization have very specific technical
goals: to reduce data redundancy and make explicit the 1:1 and M:1 relationships
within the data that can be enforced by relational database management systems. ER models are
tables, attributes
become columns
typically in third
normal form (3NF)