T-SQL Query will not return 0 value for a Sum

Question

I have a query where I'm calculating Days of Therapy for medications. I want to have 0 values to show for months that have no data. Currently the query returns no record if the Sum is 0. Can't seem to figure this out. See the Query Below:

If I were to comment out identifiers related to the DOT_ALL table along with the Where Clause I get 60 rows, 1 for each month for the past 5 years. However, otherwise i get only 57 for the drug in the Where Clause since there are not DOTs for Aug 2016, April 2016 and Jan 2015.

Thanks in advance.

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SELECT
    AMS.[Medication Name]
    , SUM(AMS.DOT) AS DOT
    ,  PD.[Patient Days]
    , PD.[Month_Name]
    , PD.[Fiscal_Month]
    , PD.[Accounting_Year]
    , PD.[Year]

FROM
    DW_PROD.dbo.Patient_Days_By_Month PD 
    Left JOIN [DW_PROD].[dbo].[DOTS_All] AMS ON (PD.Month_Name = AMS.Month AND PD.Year = AMS.Year)

WHERE
    [Medication Name] = 'CEFUROXIME'

GROUP BY
    AMS.[Medication Name]
    , PD.[Patient Days]
    , PD.[Month_Name]
    , PD.[Fiscal_Month]
    , PD.[Accounting_Year]
    , PD.[Year]

ORDER BY
    ACCOUNTING_YEAR
    ,FISCAL_MONTH

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Answer 1

This may be the cheapest solution, under the assumption that Patient_Days_By_Month is already some kind of calendar.

SELECT
    'CEFUROXIME' AS [Medication Name],
    SUM(AMS.DOT) AS DOT,
    PD.[Patient Days],
    PD.[Month_Name],
    PD.[Fiscal_Month],
    PD.[Accounting_Year],
    PD.[Year]
FROM DW_PROD.dbo.Patient_Days_By_Month PD 
LEFT JOIN [DW_PROD].[dbo].[DOTS_All] AMS 
    ON PD.Month_Name = AMS.Month 
    AND PD.Year = AMS.Year
    AND [Medication Name] = 'CEFUROXIME'
GROUP BY
    PD.[Patient Days],
    PD.[Month_Name],
    PD.[Fiscal_Month],
    PD.[Accounting_Year],
    PD.[Year]
ORDER BY
    PD.ACCOUNTING_YEAR,
    PD.FISCAL_MONTH

JOIN conditions do not need to refer to columns in other tables, they can as well contain constants .

The medication name was originally restricted in the WHERE clause - that eliminated all non-cefuroxime records from the resultset.