在sql查詢中加入聯合查詢查殺性能

Question

有沒有什麼辦法可以改善下面的查詢？

下面的查詢給出了正確的結果，但是會影響性能。

Answer 1

也許這樣？

Select 
    ID, 
    max(case when MYDATE = '20160105 10:00:00' then PRICE else 0 end), 
    max(case when MYDATE = '20140805 10:00:00' then PRICE else 0 end), 
    max(case when MYDATE = '20170215 10:00:00' then PRICE else 0 end) 
FROM 
(
    select ID, PRICE FROM TABLE1 WHERE MYDATE in ('20160105 10:00:00', '20140805 10:00:00', '20170215 10:00:00') 
    UNION ALL 
    SELECT ID, PRICE FROM TABLE2 WHERE MYDATE in ('20160105 10:00:00', '20140805 10:00:00', '20170215 10:00:00') 
) A 
group by ID 

猜測會更容易些，如果你提供的任何例子...

Answer 2

你可以避開（），使用A.ID的加入，這可能會產生一些好處的性能

Select A.ID, A.[2016 Price], B.[2014 Price], C.[2017 Price] 
FROM 

    (
    select ID, PRICE [2016 Price] FROM TABLE1 WHERE MYDATE = 2016-01-05 10:00:00 
    UNION 
    SELECT ID, PRICE FROM TABLE2 WHERE MYDATE = 2016-01-05 10:00:00 
) A 

INNER JOIN 

    (
    select ID, PRICE [2014 Price] FROM TABLE1 WHERE MYDATE = 2014-08-05 10:00:00 
    UNION 
    SELECT ID, PRICE FROM TABLE2 WHERE MYDATE = 2014-08-05 10:00:00 
) B ON A.ID =B.ID 

INNER JOIN 

    (
    select ID, PRICE [2017 Price] FROM TABLE1 WHERE MYDATE = 2017-02-15 10:00:00 
    UNION 
    SELECT ID, PRICE FROM TABLE2 WHERE MYDATE = 2017-02-15 10:00:00 
) ON A.ID =C.ID