得到行的數量有限，每聯接的列

Question

我有以下SQL語句現在：

SELECT m . * 
FROM newsletter_mail_list m 
INNER JOIN (
    SELECT n.id 
    FROM newsletter n 
    GROUP BY n.customer_id 
    ORDER BY n.id ASC 
) b ON m.newsletter_id = b.id 
WHERE m.sent_date IS NULL 

郵件列表表

+--------+-------+--------+--------+-----+ 
| id | newsletter_id | email | sent_date | 
+--------+-------+--------+--------+-----+ 
| 1 | 8    | abcd | today  | 
| 2 | 8    | cfdf | NULL  | 
| 3 | 8    | afdg | NULL  | 
| 4 | 9    | zfbh | NULL  | 
| 5 | 9    | eerg | NULL  | 
| 6 | 9    | ertg | NULL  | 
| 7 | 9    | zfbh | NULL  | 
| 8 | 9    | eerg | NULL  | 
| 9 | 9    | ertg | NULL  | 
| 10 | 9    | zfbh | NULL  | 
| 11 | 9    | eerg | NULL  | 
| 12 | 9    | ertg | NULL  | 
| 13 | 9    | zfbh | NULL  | 
| 14 | 9    | eerg | NULL  | 
| 15 | 9    | ertg | NULL  | 
| 16 | 9    | zfbh | NULL  | 
| 17 | 9    | eerg | NULL  | 
| 18 | 9    | ertg | NULL  | 
+--------+-------+--------+--------+-----+ 

所需的結果

+--------+-------+--------+--------+-----+ 
| id | newsletter_id | email | sent_date | 
+--------+-------+--------+--------+-----+ 
| 2 | 8    | cfdf | NULL  | 
| 3 | 8    | afdg | NULL  | 
| 4 | 9    | zfbh | NULL  | 
| 5 | 9    | eerg | NULL  | 
| 6 | 9    | ertg | NULL  | 
| 7 | 9    | zfbh | NULL  | 
| 8 | 9    | eerg | NULL  | 
| 9 | 9    | ertg | NULL  | 
| 10 | 9    | zfbh | NULL  | 
| 11 | 9    | eerg | NULL  | 
| 12 | 9    | ertg | NULL  | 
| 13 | 9    | zfbh | NULL  | 
+--------+-------+--------+--------+-----+ 

這一個給我所有mail_list行，其中sent_date爲NULL，並且外部時事通訊是客戶中最早的行。

現在我只想要最多10個mail_list行返回由子查詢返回的每個時事通訊。

實施例：

現在我得到21分的結果通過該SQL，屬於該通訊8 3個mail_lists和18個結果簡訊9. 但結果簡訊的數目9應該只有10個。

我怎麼能用SQL來做到這一點？這甚至有可能嗎？

Answer 1

如：

SELECT x.* 
    FROM mail_list x 
    JOIN mail_list y 
    ON y.newsletter_id = x.newsletter_id 
    AND y.id <= x.id 
WHERE x.sent_date IS NULL 
GROUP 
    BY newsletter_id 
    , id 
HAVING COUNT(*) <= 10; 

如果性能是一個問題，然後用變量的解決方案會更快...

如：

SELECT id 
    , newsletter_id 
    , email 
    FROM 
    (SELECT x.* 
      , CASE WHEN @prev=newsletter_id 
        THEN @i:=@i+1 
        ELSE @i:=1 END i 
      , @prev:=newsletter_id prev 
     FROM mail_list x 
      , (SELECT @prev:='',@i:=0) vars 
     WHERE sent_date IS NULL 
     ORDER 
      BY newsletter_id, id 
    ) a 
WHERE i <= 10;