Kueri tidak lengkap - MYSQL

Sepertinya Anda perlu menambahkan fungsi agregat:

SELECT CD_NO, purchase_date, Count(purchase_date) totalSales
FROM Artist
GROUP BY purchase_date
ORDER BY CD_NO

MySQL memungkinkan Anda untuk membatasi jumlah bidang yang Anda GROUP BY jadi ini di atas hanya pengelompokan pada purchase_date .

Di RDBMS lain, Anda bisa menggunakan subquery:

SELECT a1.CD_NO, a1.purchase_date, a2.TotalSalesByDate
FROM Artist a1
INNER JOIN
(
    SELECT count(purchase_date) TotalSalesByDate, purchase_date
    FROM Artist
    GROUP BY purchase_date
) a2
   on a1.purchase_date= a2.purchase_date
ORDER BY a1.CD_NO

Sunting:Berdasarkan pembaruan Anda, Anda seharusnya dapat menggunakan sesuatu yang mirip dengan ini:

SELECT a1.CD_NO, date_format(a1.purchase_date, '%Y-%m-%d') MostSales
FROM Artist a1
INNER JOIN
(
    SELECT count(purchase_date) TotalSalesByDate, purchase_date
    FROM Artist
    GROUP BY purchase_date
) a2
   on a1.purchase_date= a2.purchase_date
group by a1.CD_NO, a1.purchase_date
having max(TotalSalesByDate) = (select max(Total) totalsales
                                 from
                                 (
                                    SELECT cd_no, purchase_date, Count(purchase_date) Total
                                    FROM Artist a1
                                    GROUP BY cd_no, purchase_date
                                 ) src
                                 where a1.cd_no = src.cd_no
                                 GROUP BY cd_no);

Lihat SQL Fiddle dengan Demo

Hasil:

| CD_NO |  MOSTSALES |
----------------------
|     1 | 2011-12-30 |
|     2 | 2012-03-22 |
|     3 | 2012-04-22 |
|     3 | 2012-04-24 |