使用 WHILE 循环优化多项选择

Question

我需要运行下面 SQL Server 2008 查询的更大版本（SELECT 的 42 次重复，与下面显示的 4 次重复并列）。在我看来，它并不是很优化，也不是使用 while 循环的理想选择，但经过数小时的搜索和尝试后，我无法让它工作。任何人都可以建议更优化的查询来完成我需要的吗？

谢谢

SELECT 
    t1.Occupied as "1", t2.Occupied as "2", 
    t3.Occupied as "3", t4.Occupied as "4"
FROM 
    (SELECT 
         COUNT(function_id) AS Occupied
     FROM 
         ev_functions 
     WHERE 
         room_id = 22 
         AND DATEADD(HOUR,3,GETDATE()) > from_date_time 
         AND GETDATE() < to_date_time 
         AND function_status_id = 6) t1,
    (SELECT 
         COUNT(function_id) AS Occupied
     FROM 
         ev_functions 
     WHERE 
         room_id = 1 
         AND DATEADD(HOUR,3,GETDATE()) > from_date_time 
         AND GETDATE() < to_date_time 
         AND function_status_id = 6) t2,
    (SELECT 
         COUNT(function_id) AS Occupied
     FROM 
         ev_functions 
     WHERE 
         room_id = 1 
         AND DATEADD(HOUR,3,GETDATE()) > from_date_time 
         AND GETDATE() < to_date_time 
         AND function_status_id = 6) t3,
    (SELECT 
         COUNT(function_id) AS Occupied
     FROM 
         ev_functions 
     WHERE 
         room_id = 1 
         AND DATEADD(HOUR,3,GETDATE()) > from_date_time 
         AND GETDATE() < to_date_time 
         AND function_status_id = 6) t4

Answer 1

在您的情况下，您正在多次访问 Same table 以进行一小部分更改

使用 SUM 和 CASE 来避免这种情况并提高可读性

sum(case when room_id = 1 then 1 else 0 end ) as room1,
sum(case when room_id = 22 then 1 else 0 end )..
.....
and so on
from ev_functions 
where  DATEADD(HOUR,3,GETDATE()) > from_date_time 
and GETDATE() < to_date_time 
and function_status_id = 6

使用 WHILE 循环优化多项选择

Optimising multiple selects with a WHILE loop

sql-server

select

while-loop

multiple-select-query