In SQL Server the query optimizer uses a cardinality estimator to answer data SELECTIVITY questions like the ones below. The optimizer uses this cardinality data, which relies heavy on statistics to get the answers and calculate a cost estimate. It takes the #of Rows to Satisfy a Predicate/Total # of Input Rows.
source https://www.c-sharpcorner.com/article/5-ways-to-examine-cardinality-metrics/
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