Mark Kirkwood <markir@xxxxxxxxxxxxxxx> writes:
> It is a bit confusing - '(distinct) cardinality' might be a better
> heading for their 'cardinality' column!
The usual mathematical meaning of "cardinality" is "the number of
members in a set". That isn't real helpful for the point at hand,
because the mathematical definition of a set disallows duplicate
members, so if you're dealing with non-unique values you could argue it
either way about whether to count duplicates or not. However, I read in
the SQL99 spec (3.1 Definitions)
d) cardinality (of a value of a collection type): The number of
elements in that value. Those elements need not necessarily have
distinct values.
so ... as all too often ... the mysql boys have not got a clue about
standards compliance. They are using this term in the opposite way
from how the SQL committee uses it.
regards, tom lane
