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cardinality ; what is

snippet from reference

The term cardinality actually has two different meanings
depending on the context of it’s usage
one meaning is in the context of data modeling
and the other meaning is in the context of SQL statements.
.
.
when the word cardinality is used in the context of data modeling,
it simply refers to the relationship
that one table can have with another table.
In SQL, the cardinality of a column in a given table
refers to the number of unique values
that appear in the table for that column.
So, remember that the cardinality is a number. 

reference

Cardinality in SQL