如何在sql server中替換存儲過程中的union

Question

我的存儲過程具有以下查詢。有什麼方法可以將聯合換成一個查詢或其他方式來優化它。

Select col1,col2 
from logs lg 
inner join EmployeeType_1 et1 on et1.empID=lg.userID 
where lg.usertype=1 

Union 

Select col1,col2 
from logs lg 
inner join EmployeeType_2 et2 on et2.empID=lg.userID 
where lg.usertype=2 

Union 

Select col1,col2 
from logs lg 
inner join EmployeeType_3 et3 on et13.empID=lg.userID 
where lg.usertype not in (1,2) 

Answer 1

據推測，col1和col2來自logs。這裏有一種方法：

Select col1,col2 
from logs lg 
where (lg.usertype = 1 and exists (select from EmployeeType_1 et1 where et1.empID = lg.userID)) and 
     (lg.usertype = 2 and exists (select from EmployeeType_2 et1 where et1.empID = lg.userID)) and 
     (lg.usertype not in (1, 2) and exists (select from EmployeeType_3 et1 where et1.empID = lg.userID)) 

Answer 2

一個LEFT JOIN應該做的伎倆：

SELECT 
    lg.col1, 
    Coalesce(et1.empID, et2.empID, et3.empID) As col2 
FROM 
    logs As lg 
    LEFT JOIN EmployeeType_1 As et1 
    ON et1.empID = lg.userID And lg.usertype = 1 
    LEFT JOIN EmployeeType_2 As et2 
    ON et2.empID = lg.userID And lg.usertype = 2 
    LEFT JOIN EmployeeType_3 As et3 
    ON et3.empID = lg.userID And lg.usertype Not In (1, 2) 

然而，因爲你的例子只是從三個EmployeeType_X表檢索empID列，該列始終等於logs表中的userID列，則不需要加入表格：

SELECT 
    lg.col1, 
    lg.userID As col2 
FROM 
    logs As lg