Why can't you mix Aggregate values and Non-Aggregate values in a single SELECT?

Question

I know that if you have one aggregate function in a SELECT statement, then all the other values in the statement must be either aggregate functions, or listed in a GROUP BY

Answer 1

Think about it this way: when you call COUNT without grouping, it "collapses" the table to a single group making it impossible to access the individual items within a group in a select clause.

You can still get your result using a subquery or a cross join:

    SELECT p1.Name, COUNT(p2.Name) AS Surname FROM People p1 CROSS JOIN People p2 GROUP BY p1.Name

    SELECT Name, (SELECT COUNT(Name) FROM People) AS Surname FROM People