What percentage of people named Mary who wants Receive Email promotions of AdventureWorks and selected partners are store contacts?
wants Receive Email promotions of AdventureWorks and selected partners refers to EmailPromotion = 2; store contact refers to PersonType = 'SC'; percentage = DIVIDE(count(BusinessEntityID(FirstName = 'Marry'&EmailPromotion = '2')),count(BusinessEntityID)))

SELECT CAST(SUM(CASE WHEN EmailPromotion = 2 THEN 1 ELSE 0 END) AS REAL) * 100 / SUM(CASE WHEN PersonType = 'SC' THEN 1 ELSE 0 END) FROM Person WHERE FirstName = 'Mary'