如何改進此查詢？

以下查詢工作，但沒有特別快：如何改進此查詢？

select distinct controlid, (
       select max(Orderno) From my_table 
       Where ControlID = #param1# 
        and a = 3 and b = 13 and c = 0 and d = 0 
      ) colA, (
       Select max(Orderno) From my_table 
       Where ControlID = #param1# 
        and a = 2 
      ) colB, (
       Select max(Orderno) From my_table 
       Where ControlID = #param1# 
        and a = 1 and b = 14 and e = 1 
      ) colC, (
       Select max(Orderno) From my_table 
       Where ControlID = #param1# 
        and a = 3 and b = 13 and e = 1 and c = 0 and d = 0 
      ) colD, (
       Select a From my_table 
       Where ControlID = #param1# 
        and Orderno = #param2# 
      ) colE 
      from my_table 
      where controlid = #param1#

被查詢該表具有超過300K行，並且行的特定控件ID（distinct子句前）計從1變化到61。分析器說響應時間是234/42毫秒。表格中沒有索引。

很顯然，缺乏索引是一個問題，但除此之外，任何人都可以推薦一種更有效的方式來編寫這個查詢嗎？

來源

2013-03-06 earachefl

使用聚合而不是子查詢：

select distinct controlid, 
    max (case when a=3 and b=13 and c=0 and d= 0 then OrderNo end) as colA, 
    max (case when a=2       then OrderNo end) as colB, 
    max (case when a=1 and b=14 and e=1   then OrderNo end) as colC, 
    max (case when a=3 and b=13 and e=1 and c=0 and d=0 then OrderNo end) as colD, 
    max (case when OrderNo=#param2# then a end) as colE 
from my_table 
where controlid = #param1# 
group by controlid

我不知道哪個RDBMS使用，所以case when結構可能需要進行修改，以當地的方言。這一個應該是有效的MSSQL

更新：我的眼鏡不能正常工作，所以我沒有注意到ORACLE標記。尷尬......

來源

2013-03-06 19:26:18 alzaimar

可能是oracle，由於標籤..？ – Mithon 2013-03-06 19:27:39

爲了在子查詢沒有找到任何內容時保持相同的結果，else子句應該在上面爲null。此外，case表達式的默認值爲null，所以'else null'不需要，但有時用於顯示intent。 – 2013-03-06 19:34:47

你是絕對正確的。我會糾正我的嘗試。 – alzaimar 2013-03-06 19:39:21

你應該能夠避免撞到桌子多次，做一個表掃描

SELECT controlID, 
     MAX(CASE WHEN a = 3 
        AND b = 13 
        AND c = 0 
        AND d = 0 
       THEN orderNo 
       ELSE NULL 
       END) colA, 
     MAX(CASE WHEN a = 2 
       THEN orderNo 
       ELSE NULL 
       END) colB, 
     MAX(CASE WHEN a = 1 
        AND b = 14 
        AND e = 1 
       THEN orderNo 
       ELSE NULL 
       END) colC, 
     MAX(CASE WHEN a = 3 
        AND b = 13 
        AND e = 1 
        AND c = 0 
        AND d = 0 
       THEN orderNo 
       ELSE NULL 
       END) colD, 
     MAX(CASE WHEN orderNo = #param2# 
       THEN a 
       ELSE NULL 
       END) colE 
    FROM my_table 
WHERE controlId = #param1# 
GROUP BY controlID

當然，它很可能最終會被更有效地對錶相應的索引並做多個索引掃描。不知道你的數據，很難知道。

來源

2013-03-06 19:28:15

如何改進此查詢？

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