Question

我无法确切地查明为什么查询＃1返回的记录少于查询＃2：

QUERY＃1

with
cte_biggie ([NPI],[Year Entered],[Month Entered],[Count],[Practice Name],[MLIS Code],[Practice Code],
        [Physician],[Sales Rep],[Date Established],[Address],[Address2],[City],[State],[Status]) as (
select c.npi, DATEPART(yy, [DATE entered])  , 
CASE WHEN DATEPART(mm, [DATE entered]) = 01 THEN 'Jan' 
           WHEN DATEPART(mm, [DATE entered]) = 02 THEN 'Feb'
           WHEN DATEPART(mm, [DATE entered]) = 03 THEN 'Mar'
           WHEN DATEPART(mm, [DATE entered]) = 04 THEN 'Apr'
           WHEN DATEPART(mm, [DATE entered]) = 05 THEN 'May'
           WHEN DATEPART(mm, [DATE entered]) = 06 THEN 'Jun'
           WHEN DATEPART(mm, [DATE entered]) = 07 THEN 'Jul'
           WHEN DATEPART(mm, [DATE entered]) = 08 THEN 'Aug'
           WHEN DATEPART(mm, [DATE entered]) = 09 THEN 'Sep'
           WHEN DATEPART(mm, [DATE entered]) = 10 THEN 'Oct'
           WHEN DATEPART(mm, [DATE entered]) = 11 THEN 'Nov'
           WHEN DATEPART(mm, [DATE entered]) = 12 THEN 'Dec'
           END
           ,COUNT([specimen id])  ,[practice name],b.[mlis practice id],a.[practice code],[Requesting Physician],c.salesrep,
   c.dateestablished , c.practiceaddress1, c.practiceaddress2,c.practicecity,c.practicestate,
    b.[Active Inactive]
from quicklabdump a
    inner join qlmlismapping b
    on (b.[practice code] = a.[practice code])
    inner join PracticeandPhysician c
    on (a.[Requesting Physician]=c.doctorfirstname+' '+c.DOCTORLASTNAME
        and a.[practice code]=c.practicecode)
where   ( [Date Entered] >= '20100101' AND [Date Entered] < '20120101')


group by DATEPART(yy, [DATE entered]), DATEPART(mm, [DATE entered]), a.[practice name],b.[mlis practice id],a.[practice code],
    a.[Requesting Physician],c.salesrep,c.dateestablished, c.practicecity,c.practicestate,c.npi,c.practiceaddress1 ,c.practiceaddress2,
    b.[Active Inactive]


)

select [Original December Count]=SUM([count])  from cte_biggie
where [Year Entered]=2011
and [Month Entered]='Dec'

结果是

而这个查询：

QUERY# 2

;with
cte_biggie2 ([Year Entered],[Month Entered],[Count]) as (
select  DATEPART(yy, [DATE entered])  , 
CASE WHEN DATEPART(mm, [DATE entered]) = 01 THEN 'Jan' 
           WHEN DATEPART(mm, [DATE entered]) = 02 THEN 'Feb'
           WHEN DATEPART(mm, [DATE entered]) = 03 THEN 'Mar'
           WHEN DATEPART(mm, [DATE entered]) = 04 THEN 'Apr'
           WHEN DATEPART(mm, [DATE entered]) = 05 THEN 'May'
           WHEN DATEPART(mm, [DATE entered]) = 06 THEN 'Jun'
           WHEN DATEPART(mm, [DATE entered]) = 07 THEN 'Jul'
           WHEN DATEPART(mm, [DATE entered]) = 08 THEN 'Aug'
           WHEN DATEPART(mm, [DATE entered]) = 09 THEN 'Sep'
           WHEN DATEPART(mm, [DATE entered]) = 10 THEN 'Oct'
           WHEN DATEPART(mm, [DATE entered]) = 11 THEN 'Nov'
           WHEN DATEPART(mm, [DATE entered]) = 12 THEN 'Dec'
           END
           ,COUNT([specimen id]) 
from quicklabdump 
where   ( [Date Entered] >= '20100101' AND [Date Entered] < '20120101')


group by DATEPART(yy, [DATE entered]), DATEPART(mm, [DATE entered])

)

select [Original December Count2]=SUM([count])  from cte_biggie2
where [Year Entered]=2011
and [Month Entered]='Dec'

正在回归：

我不明白这是怎么回事，因为我在两个查询中都返回相同的内容，我只是选择在第一个列中显示更多列。

似乎我正在执行的join实际上限制了结果。为什么会这样？

Answer 1

是的，第一个查询中的inner join子句正在产生差异。使用内部联接，结果集将仅包含左侧表中的行，右侧表中存在匹配的行。

您可以使用left join包含左侧表格中的所有行。但是，即使使用左连接，最终输出中的行数也可能会受到影响。

修改

以下是内部联接如何限制结果集中行的简单示例：

Authors: Author_ID Name --------- -------- 1 "Fred" 2 "Susan" 3 "Bob" Posts: Post_ID Author_ID Abstract --------- ---------- ---------- 1 1 "Joel Spolsky is my hero..." 2 2 "My co-worker is crazy..."

仅从Authors中选择（没有条件）返回所有行（显然）。但是使用内部联接从Authors和Posts查询，如下所示：

SELECT a.Name, p.Abstract FROM Authors a JOIN Posts p ON p.Author_ID = a.Author_ID

产生这个结果：

Name Abstract -------- ----------- "Fred" "Joel Spolsky is my hero..." "Susan" "My co-worker is crazy..."

请注意，即使我们查询Authors表，“Bob”也没有出现在结果中......因为他没有创作过任何帖子（显然）。如果我们将连接更改为左连接，如下所示：

SELECT a.Name, p.Abstract FROM Authors a LEFT JOIN Posts p ON p.Author_ID = a.Author_ID

结果将如下所示：

Name Abstract -------- ----------- Fred "Joel Spolsky is my hero..." Susan "My co-worker is crazy..." Bob NULL

注意：我从连接中的第二个表（Abstract）返回一列（Posts）这一事实对返回的行数没有影响。

两个查询返回不同数量的记录

1 个答案: