Cardinalities and ratios

Another enhancement to the logical data model is the addition of cardinalities and ratios for each attribute. Cardinality is the number of unique elements for an attribute and ratios are the ratios between the cardinalities of related attributes.

Cardinalities help the database administrator estimate the size of the data warehouse and help project designers determine the best paths for users to navigate through the data using hierarchies in MicroStrategy, which are discussed in Creating Hierarchies to Organize and Browse Attributes. Ratios can be particularly helpful when trying to decide where creating aggregate tables will be most effective. This additional information can be invaluable to database administrators and project designers.

The following diagram shows a logical data model which includes cardinalities and ratios. Note the cardinalities in the upper right corner of each attribute rectangle and the ratios next to some of the relationships between attributes. Also note that the cardinality of some attributes such as Date of Birth are unknown; this is because this information varies and is unpredictable. For example, it is impossible to determine how many customers have different dates of birth in the warehouse.