is it possible to get the number of rows of a table?

I would like to compare the number of rows
of one table and of another and use it in
a query like this:
SELECT * FROM xxxx
WHERE "number of rows of table xxxx"
EQUALS
"number of rows of table yyyy"
i.e. I only want get a query result if the tables
have the same number of rows.
Is there a function or a way to do this ?

Thanks,
Frederick

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