按小時分組時，應返回0。這個怎麼做？

Question

我有類似這樣的一個collcation：

http://sqlfiddle.com/#!2/fecbc/3

我的問題是，當我使用group by HOUR(date)小時像2，3和4出現，但與0我想。

其實他們消失的結果。

我該如何執行此操作？

什麼是簡單的方法來做到這一點？

有沒有辦法避免24 ifs陳述？

我們可以避免創建另一個表嗎？

Answer 1

您需要一個包含24小時的表格，然後JOIN。如果您無法創建表格，則可以在查詢中創建它。 See this demo。

SELECT a.hr, IFNULL(b.hourCount, 0) hourCount 
FROM 
    (SELECT 1 AS hr 
    UNION ALL 
    SELECT 2 
    UNION ALL 
    SELECT 3 
    UNION ALL 
    SELECT 4 
    UNION ALL 
    SELECT 5 
    UNION ALL 
    SELECT 6 
    UNION ALL 
    SELECT 7 
    UNION ALL 
    SELECT 8 
    UNION ALL 
    SELECT 9 
    UNION ALL 
    SELECT 10 
    UNION ALL 
    SELECT 11 
    UNION ALL 
    SELECT 12 
    UNION ALL 
    SELECT 13 
    UNION ALL 
    SELECT 14 
    UNION ALL 
    SELECT 15 
    UNION ALL 
    SELECT 16 
    UNION ALL 
    SELECT 17 
    UNION ALL 
    SELECT 18 
    UNION ALL 
    SELECT 19 
    UNION ALL 
    SELECT 20 
    UNION ALL 
    SELECT 21 
    UNION ALL 
    SELECT 22 
    UNION ALL 
    SELECT 23 
    UNION ALL 
    SELECT 0) a 
LEFT JOIN 
    (SELECT COUNT(id) AS hourCount, HOUR(date) AS hr 
    FROM `testing` 
    GROUP BY HOUR(date)) b ON b.hr = a.hr 

Answer 2

如果您想要前導零，請在您的查詢中使用lpad。

SELECT 
    COUNT(id) AS hourCount, 
    lpad(HOUR(date),2,'0') AS hour 
FROM 
    testing 
GROUP BY 
    HOUR(date) 
; 

http://sqlfiddle.com/#!2/fecbc/5/0

Answer 3

如果您不希望創建這樣的表，你可以使用從0到23枚舉小時的子查詢：

SELECT 
    v_hours.hr as hour, 
    COUNT(id) AS hourCount 
FROM 
    (select 0 as hr union all 
    select 1 as hr union all 
    select 2 as hr union all 
    select 3 as hr union all 
    select 4 as hr union all 
    select 5 as hr union all 
    select 6 as hr union all 
    select 7 as hr union all 
    select 8 as hr union all 
    select 9 as hr union all 
    select 10 as hr union all 
    select 11 as hr union all 
    select 12 as hr union all 
    select 13 as hr union all 
    select 14 as hr union all 
    select 15 as hr union all 
    select 16 as hr union all 
    select 17 as hr union all 
    select 18 as hr union all 
    select 19 as hr union all 
    select 20 as hr union all 
    select 21 as hr union all 
    select 22 as hr union all 
    select 23 as hr 
) v_hours 
LEFT JOIN testing 
    ON HOUR(date) = v_hours.hr 
GROUP BY 
    v_hours.hr 
ORDER BY v_hours.hr