用UNION关键字查询在mysql中运行速度非常慢

Question

为什么在UNION关键字查询中运行非常慢的mysql？

我已经使用UNION关键字创建了一个查询，但每次执行它时，显示输出都需要很长时间。

这是我使用的查询。

SELECT process_trained AS Training_Title, 
    org_party AS Organizing_Party, 
    duration AS Duration, 
    Concat(Date_format(sptrain_from, '%d-%b-%y'), " to ", 
    Date_format(sptrain_to, '%d-%b-%y'), "") AS SpecialTraining_Date, 
    trainor AS Trainor, 
    category AS Category, 
    training_type AS Training_Type, 
    train_id, 
    Date_format(cert_date, '%d-%b-%Y') AS Date_Certified, 
    Date_format(re_certdate, '%d-%b-%Y') AS Re_Certificaton, 
    remarks, 
    filename 
FROM(SELECT *, 
       cert_date AS cdate 
     FROM tbldirtraining 
     UNION ALL 
     SELECT *, 
       sptrain_from 
     FROM tbldirtraining) jes 
WHERE emp_id = '6555' 
     AND cdate <> '' 
ORDER BY cdate; 

是否有任何可能的方法可以帮助如何更快地运行此查询。

在此先感谢。

Answer 1

试试这个

 SELECT process_trained AS Training_Title, 
org_party AS Organizing_Party, 
duration AS Duration, 
Concat(Date_format(sptrain_from, '%d-%b-%y'), " to ", 
Date_format(sptrain_to, '%d-%b-%y'), "") AS SpecialTraining_Date, 
trainor AS Trainor, 
category AS Category, 
training_type AS Training_Type, 
train_id, 
Date_format(cert_date, '%d-%b-%Y') AS Date_Certified, 
Date_format(re_certdate, '%d-%b-%Y') AS Re_Certificaton, 
remarks, 
filename 
FROM tbldirtraining 
WHERE emp_id = '6555' 
AND cert_date <> '' 
ORDER BY cert_date desc,sptrain_from asc;