我怎么能改变这个查询

Question

SELECT * FROM (
      SELECT t1.eventId,t1.start_date,t1.end_date, COUNT(*) pos FROM events t1
        LEFT JOIN events t2 ON t1.start_date = t2.start_date AND t1.eventId <= t2.eventId
        WHERE t1.start_date BETWEEN  '2012-12-18' AND '2012-12-24'
      GROUP BY
        t1.eventId,t1.start_date
      ORDER BY 
        t1.start_date,pos ASC
      ) t
    WHERE
      pos <= 3;

为什么开始日期2012-12-21和2012-12-24不存在，

如果start_date不存在，那么我需要

的空值或空值

注意：我的数据库中没有21和24 ..的记录，但我的查询中需要一个空值

Answer 1

请使用以下查询并告诉我结果

SELECT * FROM (
      SELECT t1.eventId,t1.start_date,t1.end_date, COUNT(*) pos FROM events t1
        LEFT JOIN events t2 ON t1.start_date = t2.start_date AND t1.eventId <= t2.eventId
        WHERE t1.start_date >=  '2012-12-18' AND  t1.start_date <='2012-12-24'
      GROUP BY
        t1.eventId,t1.start_date
      ORDER BY 
        t1.start_date,pos ASC
      ) t
    WHERE
      pos <= 3;

Answer 2

您可以使用HAVING消除外部查询：

SELECT t1.eventId,t1.start_date,t1.end_date, COUNT(*) pos FROM events t1
LEFT JOIN events t2 ON t1.start_date = t2.start_date AND t1.eventId <= t2.eventId
WHERE t1.start_date BETWEEN  '2012-12-18' AND '2012-12-24'
GROUP BY t1.eventId,t1.start_date
HAVING COUNT(*) <= 3
ORDER BY t1.start_date,pos ASC;

请注意插入此行：

HAVING COUNT(*) <= 3

这在逻辑上等同于您的外部查询正在执行的操作