如何根據多個條件選擇列？

Question

我有一個包含多個price-timestamp元組的表：

#mytable;
id;<somecolumns>;price1;timestamp1;price2;timestamp2;

我想在臨時表中導出每個價格時間戳元組，但是僅導出其時間戳在特定時間間隔內的價格：

#mytemp
id;price1;price2;

我可以通過為每個元組重復sql來實現此目的：

INSERT INTO mytemp (price1)
SELECT price1 FROM mytable WHERE timestamp1 > NOW() - INTERVAL 3 HOUR;

INSERT INTO mytemp (price2)
SELECT price2 FROM mytable WHERE timestamp2 > NOW() - INTERVAL 3 HOUR;

#repeat for all price-timestamp tuples

問題：是否可以將其優化為一個SQL查詢 ？

Answer 1

您可以在以下情況下使用案例

  SELECT
   sum(case when timestamp1 > NOW() - INTERVAL 3 HOUR then price1 else 0 end) as price1,
    sum(case when timestamp2 > NOW() - INTERVAL 3 HOUR then price2 else 0 end)
    as price2
     FROM mytable  ;

但如果要插入，只需選擇

INSERT INTO mytemp (price1,price2)
SELECT
   sum(case when timestamp1 > NOW() - INTERVAL 3 HOUR then price1 else 0 end) as price1,
    sum(case when timestamp2 > NOW() - INTERVAL 3 HOUR then price2 else 0 end)
    as price2
     FROM mytable

Answer 2

在以下case when嘗試使用該用case when

INSERT INTO mytemp (price1,price2)
SELECT max(case when timestamp1 > NOW() - INTERVAL 3 HOUR then price1 end),max(case when timestamp2 > NOW() - INTERVAL 3 HOUR then price2 end) 
FROM mytable 

Answer 3

嘗試這個

SELECT price1 , '' as price2 from testa WHERE timestamp1 > NOW() - INTERVAL 3 HOUR
UNION
SELECT '' as price1, price2  as price from testa WHERE timestamp2 > NOW() - INTERVAL 3 HOUR

Answer 4

您可以使用cross join聯接取消數據透視：

select (case when n.n = 1 then price1
             when n.n = 2 then price2
        end),
       (case when n.n = 1 then timestamp1
             when n.n = 2 then timestamp2
        end)       
from t cross join
     (select 1 as n union all select 2) n
having timestamp > now() - interval 3 hour;

但是，如果您在timestamp上具有索引，則您的方法或與union all等效的方法可能是最快的方法。