MySQL 查询 - 转换为左连接以显示零结果？

Question

我不太擅长使用联接 - 所以我有一个表，我在其中计算满足特定条件的记录数，并按周返回这些计数。问题是，我也需要计数为零的周......我试图让它与左连接一起工作，但我正在努力......任何帮助表示赞赏:(邮票是一个日期时间字段)

查询:

SELECT week(stamp), count(*) AS mycount, YEAR(stamp) as theyear
FROM merges
WHERE completed = 1
AND stamp BETWEEN '2017/4/1 00:00:00' AND '2017/6/1 00:00:00' GROUP BY week(stamp)

这将返回:

week(stamp) | mycount | theyear

15 | 21 |2017

17 | 10 |2017

18 | 62 |2017

19 | 13 |2017

20 | 76 |2017

21 | 22 |2017

注意到第 16 周缺失了吗？我需要将此结果包含在上面，例如:

16 | 0 |2017

我感谢任何帮助 - 我知道这并不太难，但我在阅读其他帖子时绞尽脑汁试图了解如何做到这一点......

Answer 1

select weekValue, yearValue, coalesce(mycount,0)
from 

( SELECT distinct week(@startDate := @startDate + Interval 1 day) as weekValue,
year(@startDate := @startDate + Interval 1 day) as yearValue
FROM 
(select 0 union all select 1 union all select 3 union all select 4 
    union all select 5 union all select 6 union all select 6 union all select 7 
    union all select 8 union all select 9) t,
(select 0 union all select 1 union all select 3 
    union all select 4 union all select 5 union all select 6 
    union all select 6 union all select 7 union all select 8 union all select 9) t2,
(select 0 union all select 1 union all select 3 union all select 4 union all select 5 union all select 6 union all select 6 union all select 7 union all select 8 union all select 9) t3, 
(select 0 union all select 1 union all select 3 union all select 4 union all select 5 union all select 6 union all select 6 union all select 7 union all select 8 union all select 9) t4, 
(SELECT @startDate := '2017-03-31 00:00:00' ) as g

where 
@startDate < '2017-06-01 00:00:00' ) as generateWeekYear left join 

(SELECT week(stamp) as theweek, count(*) AS mycount, YEAR(stamp) as theyear
FROM merges
WHERE completed = 1
AND stamp BETWEEN '2017/4/1 00:00:00' AND '2017/6/1 00:00:00' GROUP BY week(stamp) ) as actualQuery

on  generateWeekYear.weekValue = actualQuery.theweek 
and generateWeekYear.yearValue = actualQuery.theyear

让我解释一下上面的查询，

子查询generateWeekYear =这用于根据两个输入生成不同的周和年 让我们说开始日期和结束日期。开始日期应比实际开始日期少 1 天。因为如果你不这样做 减去 1 天，则可能有机会减掉 1 周。

现在您拥有需要显示的所有周和年。

现在您认为generateWeekYear 的执行时间会更长，但事实并非如此。你可以 检查这个generate an integer sequence in MySQL 。

之后，您只需将您的表格与上面的表格连接起来，就可以获得您所需的结果。