我怎么能得到0没有使用MySQL的记录？

Question

我是mySql的新手。我必须根据星期几来计算表格中的点击次数。

这里期限取决于数据库。但是如果上周没有记录，它会发生什么，它不在列表中。但我需要它作为0.那么，我该怎么做？

SELECT CASE 
    WHEN PERIOD >= NOW() - INTERVAL 4 WEEK 
       AND PERIOD < NOW() - INTERVAL 3 WEEK THEN '22-28 days ago' 

    WHEN PERIOD >= NOW() - INTERVAL 3 WEEK 
       AND PERIOD < NOW() - INTERVAL 2 WEEK THEN '15-21 days ago' 

    WHEN PERIOD >= NOW() - INTERVAL 2 WEEK 
     AND PERIOD < NOW() - INTERVAL 1 WEEK THEN '8-14 days ago' 

    WHEN PERIOD >= NOW() - INTERVAL 1 WEEK 
       THEN 'up to 7 days ago' 
      END WeekPeriod, IFNULL(count(CLICKS),0) TotalClicks 

FROM TABLE 

WHERE PERIOD >= NOW() - INTERVAL 4 WEEK 

GROUP BY CASE 
    WHEN PERIOD >= NOW() - INTERVAL 4 WEEK 
        AND PERIOD < NOW() - INTERVAL 3 WEEK THEN '22-28 days ago' 

    WHEN PERIOD >= NOW() - INTERVAL 3 WEEK 
       AND PERIOD < NOW() - INTERVAL 2 WEEK THEN '15-21 days ago' 

    WHEN PERIOD >= NOW() - INTERVAL 2 WEEK 
       AND PERIOD < NOW() - INTERVAL 1 WEEK THEN '08-14 days ago' 

    WHEN PERIOD >= NOW() - INTERVAL 1 WEEK 
       THEN '01 to 7 days ago' 

END 
ORDER BY WeekPeriod"; 

答案应该是这样的

1-7 0 
8-15 14 
16-21 34 
21-28 45 

Answer 1

SELECT uq.timespan, COALESCE(tsq.TotalClicks, 0) as Clicks FROM (
SELECT '22-28 days ago' as timespan 
union SELECT '15-21 days ago' 
union SELECT '8-14 days ago' 
union SELECT 'up to 7 days ago' 
)uq LEFT JOIN (
SELECT CASE 
    WHEN PERIOD >= NOW() - INTERVAL 4 WEEK 
       AND PERIOD < NOW() - INTERVAL 3 WEEK THEN '22-28 days ago' 
    WHEN PERIOD >= NOW() - INTERVAL 3 WEEK 
       AND PERIOD < NOW() - INTERVAL 2 WEEK THEN '15-21 days ago' 
    WHEN PERIOD >= NOW() - INTERVAL 2 WEEK 
     AND PERIOD < NOW() - INTERVAL 1 WEEK THEN '8-14 days ago' 
    WHEN PERIOD >= NOW() - INTERVAL 1 WEEK THEN 'up to 7 days ago' 
      END WeekPeriod, 
      count(CLICKS) TotalClicks 
FROM TABLE 
WHERE PERIOD >= NOW() - INTERVAL 4 WEEK 
GROUP BY WeekPeriod 
/*ORDER BY WeekPeriod -- unnecessary, GROUP BY implicits ORDER BY*/ 
)tsq ON uq.timespan = tsq.WeekPeriod