not contained in either an aggregate function or the GROUPby
Msg 8120, Level 16, State 1, Line 1
Column 'Citys.City_Name' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.
SELECT sum(Member_Info.IDCity), Citys.City_Name FROM Member_Info INNER JOIN Citys ON Member_Info.IDCity = Citys.IDCity GROUP BY Member_Info.IDCity
And how can use where