Cardinality in generated queries

In 0:1, 0 is the minimum cardinality, 1 is the maximum cardinality.

In 1:n, 1 is the minimum cardinality, n is the maximum cardinality.

A relationship with cardinality specified as 1:1 to 1:n is commonly referred to as 1 to n when focusing on the maximum cardinalities.

A minimum cardinality of 0 indicates that the relationship is optional. You specify a minimum cardinality of 0 if you want the query to retain the information on the other side of the relationship in the absence of a match. For example, a relationship between customer and actual sales may be specified as 1:1 to 0:n. This indicates that reports will show the requested customer information even though there may not be any sales data present.

Therefore a 1 to n relationship can also be specified as:

• 0:1 to 0:n
• 0:1 to 1:n
• 1:1 to 0:n
• 1:1 to 1:n

Use the Relationship impact statement in the Relationship Definition dialog box to help you understand cardinality. For example, Sales Staff (1:1) is joined to Orders (0:n).

It is important to ensure that the cardinality is correctly captured in the model because it determines the detection of fact query subjects and it is used to avoid double-counting factual data.

When generating queries, IBM Cognos software follows these basic rules to apply cardinality:

• Cardinality is applied in the context of a query.
• 1 to n cardinality implies fact data on the n side and implies dimension data on the 1 side.
• A query subject may behave as a fact query subject or as a dimensional query subject, depending on the relationships that are required to answer a particular query.

Use the Model Advisor to see an assessment of the behavior implied by cardinality in your model.

For more information, see Single Fact Query and Multiple-fact, Multiple-grain Query on Conformed Dimensions.