SQL Get distinct tax rate form a tax table - sql

Question

I am optimistic that some clever SQL author can tweak my query to give the results I am looking for. I have to extract the current tax rates for a state and distinct tax type (PST/QST/GST/HST) There is usually PST/GST or HST per state. Looking at the data table below I need to get the tax 'rate' for the distinct value of 'Shortname', 'name', and 'Effective Date'.
The desired result is this table:
EffectiveDate Rate ShortName name
2016-01-01 0.050000 AB GST
2013-04-01 0.050000 BC GST
2013-04-01 0.070000 BC PST
2013-07-01 0.050000 MB GST
2013-07-01 0.080000 MB PST
2016-07-01 0.150000 NB HST
2016-07-01 0.150000 NL HST
2016-01-01 0.150000 NS HST
2016-01-01 0.050000 NT GST
2016-01-01 0.050000 NU GST
2015-04-01 0.130000 ON HST
2016-10-01 0.150000 PE HST
2013-01-01 0.050000 QC GST
2017-03-01 0.099750 QC QST
2016-01-01 0.050000 SK GST
2017-03-01 0.060000 SK PST
2009-02-01 0.050000 YT GST
My query so far:
SELECT --TaxRates.ID, --TaxRates.StateId, TaxRateDetails.id,
TaxRateDetails.EffectiveDate, TaxRateDetails.Rate,--TaxRateDetails.TaxRateId,
States.ShortName, TaxImpositionTypes.name -- DISTINCT Name,states.ShortName, TaxRateDetails.Id,TaxImpositionTypeDetailId
FROM dbo.TaxRates
join dbo.TaxRateDetails ON TaxRateDetails.TaxRateId = TaxRates.Id
INNER JOIN States ON dbo.TaxRates.StateId = States.Id AND States.IsActive =1
INNER JOIN TaxImpositionTypeDetails ON TaxRateDetails.TaxImpositionTypeDetailId = TaxImpositionTypeDetails.Id
INNER JOIN TaxImpositionTypes ON dbo.TaxImpositionTypeDetails.TaxImpositionTypeId = TaxImpositionTypes.Id
WHERE EffectiveDate < CURRENT_TIMESTAMP
order BY ShortName
This source table created by this query is as follows:
EffectiveDate Rate ShortName name
2016-01-01 0.050000 AB GST
2009-02-01 0.050000 AB GST
2013-04-01 0.050000 BC GST
2013-04-01 0.070000 BC PST
2013-04-01 0.050000 BC GST
2013-04-01 0.070000 BC PST
2013-07-01 0.050000 MB GST
2013-07-01 0.080000 MB PST
2010-07-01 0.130000 NB HST
2016-07-01 0.150000 NB HST
2010-07-01 0.130000 NB HST
2009-02-01 13.000000 NL HST
2016-07-01 0.150000 NL HST
2010-07-01 0.130000 NL HST
2016-01-01 0.150000 NS HST
2010-07-01 0.150000 NS HST
2016-01-01 0.050000 NT GST
2009-02-01 0.050000 NT GST
2016-01-01 0.050000 NU GST
2009-02-01 0.050000 NU GST
2010-07-01 0.130000 ON HST
2009-02-01 0.050000 ON HST
2009-02-01 0.080000 ON PST
2010-03-01 0.130000 ON HST
2015-04-01 0.130000 ON HST
2015-04-01 0.000100 ON PST
2009-02-01 13.000000 PE HST
2013-04-01 0.140000 PE HST
2016-10-01 0.150000 PE HST
2013-01-01 0.050000 QC GST
2013-01-01 0.010000 QC QST
2012-01-01 0.050000 QC GST
2012-01-01 0.095000 QC QST
2017-03-01 0.099750 QC QST
2016-01-01 0.050000 SK GST
2016-01-01 0.050000 SK PST
2009-02-01 0.050000 SK GST
2009-02-01 0.050000 SK PST
2017-03-01 0.060000 SK PST
2009-02-01 0.050000 YT GST

Answer 1

If I understand the question correctly, we could assign a row number to each unique pairing of shortname and name order that pairing by effective date descending so the highest date is first. Then we only display row 1 of each unique pairing of shortname and name .
WITH CTE AS (
SELECT TaxRateDetails.EffectiveDate
, TaxRateDetails.Rate
, States.ShortName
, TaxImpositionTypes.name
, Row_number() over (partition by States.ShortName,TaxImpositionTypes.name ORDER BY TaxRateDetails.EffectiveDate DESC) RN
FROM dbo.TaxRates
INNER JOIN dbo.TaxRateDetails
ON TaxRateDetails.TaxRateId = TaxRates.Id
INNER JOIN States
ON dbo.TaxRates.StateId = States.Id
AND States.IsActive =1
INNER JOIN TaxImpositionTypeDetails
ON TaxRateDetails.TaxImpositionTypeDetailId = TaxImpositionTypeDetails.Id
INNER JOIN TaxImpositionTypes
ON dbo.TaxImpositionTypeDetails.TaxImpositionTypeId = TaxImpositionTypes.Id
WHERE EffectiveDate < CURRENT_TIMESTAMP)
SELECT * FROM CTE where RN = 1