SQL Server calculating percentage when using cross apply

Question

Using below query i am trying to get how many users are falling under which age group, for this i applied cross apply.

SELECT y.AgeDecade, COUNT(u.[UserId]) AS [TotalUsers]
FROM dbo.[User] u
    CROSS APPLY (SELECT Age = (CONVERT(int, CONVERT(char(8), GETDATE(), 112)) - CONVERT(int, CONVERT(char(8), u.DateOfBirth, 112))) / 10000) x
    CROSS APPLY (SELECT AgeDecade = CASE
                                        WHEN x.Age <= 29 THEN 20
                                        WHEN x.Age BETWEEN 30 AND 39 THEN 30
                                        WHEN x.Age BETWEEN 40 AND 49 THEN 40
                                        WHEN x.Age >= 50 THEN 50
                                        ELSE NULL
                                    END
    ) y
GROUP BY y.AgeDecade

Now besides [TotalUsers] i want to have another column [Percentage] which should give me the Percentage of that row across entire column.

I tried using 100 * COUNT(U.[UserId]) / SUM(U.[UserId]) but i see an erros stating UserId is not present in the select clause.. how do i get the percentage in this case ?

score 1 · Accepted Answer · answered Dec 17 '20 at 22:53

Use window functions:

SELECT y.AgeDecade, COUNT(*) AS [TotalUsers],
     100.0 * COUNT(*) / SUM(COUNT(*)) OVER() as [Percent]
FROM dbo.[User] u
    CROSS APPLY (SELECT Age = (CONVERT(int, CONVERT(char(8), GETDATE(), 112)) - CONVERT(int, CONVERT(char(8), u.DateOfBirth, 112))) / 10000) x
    CROSS APPLY (SELECT AgeDecade = CASE
                                        WHEN x.Age <= 29 THEN 20
                                        WHEN x.Age BETWEEN 30 AND 39 THEN 30
                                        WHEN x.Age BETWEEN 40 AND 49 THEN 40
                                        WHEN x.Age >= 50 THEN 50
                                        ELSE NULL
                                    END
    ) y
GROUP BY y.AgeDecade

Note: presumaby u.[UserId] is never NULL, so COUNT(u.[UserId]) can be simplified as COUNT(*).

SQL Server calculating percentage when using cross apply

1 Answers1