Cardinality generally refers to means two aspects in the context of databases; relationship and data cardinality.
Relationship cardinality
This refers to how two database tables are related through a key field, i.e. one-to-one, many-to-one, or many-to-many. In principle, the Qlik Engine does not care about relationship cardinality. The engine works the same way regardless of the relationship cardinality.
There is one scenario where relationship cardinality matters in QlikView; When showing frequency for keys in List boxes, as this requires identifying a table as the one where the key is a foreign key so that this table can be used to calculate the frequency
Data cardinality
This refers to how many distinct (unique) values are in a column of a database table. High cardinality refers to a high number of distinct values in relation to the total number of records in the column, while low cardinality refers to the opposite where few distinct values are repeated among a higher number of records.
References:
