[英]What kind of query to use to get this (left join not working)
table : transmission
--------------------------------------------------------
ID ReqString Timestamp Actif
------- ------------- --------------------- --------
a O21 2016-05-02 10:03:27 1
a O20 2016-05-01 11:07:47 1
a O11 2016-05-02 09:27:53 1
b O20 2016-05-02 12:27:45 1
b O21 2016-05-01 09:32:55 1
對於相同的ID,我需要檢索ReqString LIKE O2%和LIKE O1%的最新值
我已經嘗試過此LEFT JOIN。 當我在t1中有一個值時,此查詢有效,但是當我在表t1中沒有任何值時,該查詢不起作用...
SELECT t1.ReqString AS O1, t2.ReqString AS O2, t1.Timestamp AS T1, t2.Timestamp AS T2
FROM transmission t1
LEFT JOIN transmission t2 ON t2.ID = t1.ID
AND t2.ReqString LIKE 'O2%'
AND t2.Actif=1
WHERE t1.ID = 'b'
AND t1.ReqString LIKE 'O1%'
AND t1.Actif = 1
ORDER BY t1.Timestamp DESC, t2.Timestamp DESC
LIMIT 1
因此,如果我運行ID ='a'的查詢,則需要獲取
------------------------------------------------------------------------
O1 O2 T1 T2
------- --------- ----------------------- -------------------------
O11 O21 2016-05-02 09:27:53 2016-05-02 10:03:27
如果我為ID ='b'運行它,我想要的結果是
------------------------------------------------------------------------
O1 O2 T1 T2
------- --------- ----------------------- -------------------------
NULL O20 NULL 2016-05-02 12:27:45
select t1.ReqString AS O1, t2.ReqString AS O2, t1.Timestamp AS T1, t2.Timestamp AS T2 from
(SELECT ReqString , Timestamp
FROM transmission where ReqString LIKE 'O1%' AND Actif=1 and ID = 'a'
limit 1 order by Timestamp DESC)t1,
(SELECT ReqString , Timestamp
FROM transmission where ReqString LIKE 'O2%' AND Actif=1 and ID = 'a'
limit 1 order by Timestamp DESC)t2
嘗試這個:
SELECT
result1.ReqString as 'O1',
result2.ReqString as 'O2',
result1.Timestamp as 'T1',
result2.TimeStamp as 'T2'
FROM
(
SELECT
@i:=@i+1 AS rowId,
ReqString,
Timestamp
FROM transmission,(SELECT @i:=0) a
WHERE ReqString LIKE 'O1%'
AND Actif=1
AND ID = 'a'
LIMIT 1
ORDER BY Timestamp DESC
) as result1
LEFT JOIN
(
SELECT
@j:=@j+1 AS rowId,
ReqString,
Timestamp
FROM transmission,(SELECT @j:=0) a
WHERE ReqString LIKE 'O2%'
AND Actif=1
AND ID = 'a'
LIMIT 1
ORDER BY Timestamp DESC
) as result2
ON result1.rowId = result2.rowId;
我覺得左聯接可能不是您想要的。 無論result1中是否有值,這都應該產生期望的結果。 如果沒有提供所需的結果,請告訴我哪里出了問題。
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