T SQL Coalesece instead of Case

This query calculates the AdjValue by subtracting TaxDepr from DpreciableBasis. To get the TaxDepr I call a function passing an EquipOid and a date. The date is converted to the 1st day of the next month. If the result of the function is 0.00 then I grab the value from another table. Can I use the Coalesce expression instead of a Case statement? How would this be done?    Thanks.

                      ,AdjValue = isnull([ED1].TaxDepreciationDepreciableBasis,0)-
                              when      isnull(dbo.GetEquipmentTotalTaxDeprAmtFN
                                    (E1.EquipmentOid,CONVERT(varchar(7), DATEADD(m, 1, ff1.DateFunded), 23) + '-01'),0.00) = 0.00
                              then isnull([ED1].TaxDepreciationLifetime,0.00)      
                                    (E1.EquipmentOid,CONVERT(varchar(7), DATEADD(m, 1, ff1.DateFunded), 23) + '-01'))
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Shaun KlineLead Software EngineerCommented:
You shouldn't need the Coalesce function. The IsNull function should be sufficient:
isnull(dbo.GetEquipmentTotalTaxDeprAmtFN(E1.EquipmentOid,CONVERT(varchar(7), DATEADD(m, 1, ff1.DateFunded), 23) + '-01'), dbo.GetEquipmentTotalTaxDeprAmtFN(E1.EquipmentOid,CONVERT(varchar(7), DATEADD(m, 1, ff1.DateFunded), 23) + '-01'))

If you have a concern that the first part of the ISNULL function will return 0.00, you can wrap that part up in the NullIF function.
Kyle AbrahamsSenior .Net DeveloperCommented:
can GetEquipmentTotalTaxDeprAmtFN actually return 0?  (not null)

Coalesce will only help you if you're returning null values.
ryannAuthor Commented:
My Case statement works fine. A person told me that Coalesce would be another option I could use and would make the code easier to read. I am just curious how I would code this query using coalesce instead of a Case statement.
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Shaun KlineLead Software EngineerCommented:
Missed the second IsNULL.

You can do this:
                                     (E1.EquipmentOid,CONVERT(varchar(7), DATEADD(m, 1, ff1.DateFunded), 23) + '-01'), [ED1].TaxDepreciationLifetime,0.00)
Kyle AbrahamsSenior .Net DeveloperCommented:
The above assumes GetEquipmentTotalTaxDeprAmtFN will only return nulls and not 0.  If GetEquipmentTotalTaxDeprAmtFN can return a [ED1].TaxDepreciationLifetime will not be used as in your case.

I guess you could combine with a nullif:

                                     (E1.EquipmentOid,CONVERT(varchar(7), DATEADD(m, 1, ff1.DateFunded), 23) + '-01'),0), [ED1].TaxDepreciationLifetime,0.00)

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ryannAuthor Commented:
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Microsoft SQL Server 2008

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