Question

I am having trouble pinpointing exactly why query #1 is returning less records than query #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'

the result is

79009

whereas this query:

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'

is returning:

108357

I do not understand how this can be happening since I am returning THE SAME THING in both queries, I’m just choosing to display more columns in the first one.

It seems like the join that i am performing is actually limiting the results. Why does this happen?

Answer 1

Yes, it’s the inner join clauses in the first query that are making the difference. With an inner join, the result set will only include rows from the left-hand table where there are matching rows in the right-hand table.

You can include all rows from the left-hand table with a left join. Even with a left join, though, the number of rows in the final output is likely to be affected.

EDIT

Here’s a simple example of how an inner join can limit the rows in the resultset:

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..."

Selecting from only Authors (with no condition) returns all rows (obviously). But querying from both Authors and Posts with an inner join, like this:

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

produces this result:

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

Notice that even though we are querying the Authors table, “Bob” does not appear in the results…because he hasn’t authored any posts (apparently). If we change the join to a left join, like this:

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

the result will look like this:

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

Note: The fact that I am returning a column (Abstract) from the second table in the join (Posts) makes no difference to the number of returned rows.