In SQL, Cardinality is the uniqueness of data values in a column. If cardinality is low then a column will have more duplicated values.
Database use Cardinality of a column to determine the optimal query plan for a query.
of elements in a set. Thinking in the database world, cardinality has to do with the counts in a relationship, one-to-one, one-to-many, or many-to-many.