需要優化查詢以避免子查詢在if if條件中的查詢中進行兩次求值

Question

我有一個查詢使用子查詢兩次來獲得If else條件中的結果，因爲我無法首先保存結果在條件下評估。我怎樣才能優化查詢任何幫助，我會感激。

SELECT 
    IF(
     (SELECT 
       CONVERT(IFNULL(SUM(T.hours), 0) , DECIMAL (10 , 2)) As Val 
      FROM 
       time_entries T 
        JOIN 
       Issues IU2 ON (t.issue_id = IU2.Id) 
      Where 
       IU2.id = IU.Id) <> 0, 

     (SELECT 
       CONVERT(IFNULL(SUM(T.hours), 0) , DECIMAL (10 , 2)) As Val 
      FROM 
       time_entries T 
        JOIN 
       Issues IU2 ON (t.issue_id = IU2.Id) 
      Where 
       IU2.id = IU.Id), 
     99  
     ) AS NotCovered 
FROM 
    issues IU 
     Join 
    custom_values CV ON (IU.project_id = CV.customized_id 
     AND CV.value = '') 
where 
    IU.done_ratio <> 100; 

Answer 1

嘗試計算所有第一，並將其​​存儲在一個臨時表，事後你可以對臨時表中的IIF或CASE。