在where子句

Question

我有超过8000条记录的记录集。我有一个select子句（在where子句中），它将带回10条记录。

正在进行多少次计算？

它相当于搜索80,000（10 x 8000）条记录吗？或者它是64,000,000（8000 x 8000），因为每次它通过它再次搜索整个表来获得子集？

SQL代码如下。该代码的目的是找到过去7天内已关闭的作业，并返回所有相关代码，无论它们是否已打开。

实施例。如果作业S15195昨天关闭，那么它也应该返回X15195，SW15195，W15195C01。

代码可以运行但需要大约30分钟才能运行。然而，由于这将在一夜之间运行并通过电子邮件发送，因此它不应成为问题。

SELECT 
JOBHEADER.ORDERNUMBER, 
JOBHEADER.ORDERSTATUS, 
JOBHEADER.ORDERDESCRIPTION, 
JOBHEADER.ORDERDATE, 
JOBHEADER.CUSTOMER, 
JOBHEADER.JOBACTUALCOSTS, 
JOBHEADER.JOBCLOSEDATE, 
JOBHEADER.INVOICEDNETTAMOUNT, 
JOBHEADER.SALESPERSON, 
JOBHEADER.ADDITIONALFIELD_15 AS "PROJECT MANAGER",
Case substring(ORDERNUMBER FROM 2 FOR 1)
  when '1' then substring(ORDERNUMBER FROM 2 for 5)
  when '2' then substring(ORDERNUMBER FROM 2 for 5)
  else substring(ORDERNUMBER from 3 for 5)
End As "PROJECT NUMBER"

FROM JOBHEADER 

WHERE 
Case substring(ORDERNUMBER FROM 2 FOR 1)
  when '1' then substring(ORDERNUMBER FROM 2 for 5)
  when '2' then substring(ORDERNUMBER FROM 2 for 5)
  else substring(ORDERNUMBER from 3 for 5)
End 
IN 
(SELECT 
Case substring(ORDERNUMBER FROM 2 FOR 1)
  when '1' then substring(ORDERNUMBER FROM 2 for 5)
  when '2' then substring(ORDERNUMBER FROM 2 for 5)
  else substring(ORDERNUMBER from 3 for 5)
End As "PROJECT NUMBER"
FROM JOBHEADER 
WHERE DateDiff(Day, JOBHEADER.JOBCLOSEDATE, Cast('NOW' as Date)) <7 
)


ORDER BY JOBHEADER.CUSTOMER

select子句中的select子句是否更快或是否与记录搜索量相同？