FTT Exercise 22. Number of Users for Email Provider

WITH
    MailProviders_CTE ([Email Provider], [User])
        AS
        (
            SELECT  SUBSTRING(
                        Email,
                        CHARINDEX('@', Email) + 1,
                        LEN(Email) -
                            CHARINDEX('@', Email)
                            + 1
                        )
                     AS [Email Provider],
                     1 AS [UserUnit]
            FROM    [Users]
        )
    SELECT  [Email Provider],
            SUM([User]) AS [Number Of Users]
    FROM    MailProviders_CTE
    GROUP BY    [Email Provider]
    ORDER BY    [Number Of Users] DESC,
                [Email Provider] ASC