Trying to combine Inner Join and two where statements with a Select Max by date and save result into variable - sql

Question

I'm trying to use a inner join to combine 3 tables into one and pull 2 variables based on the most recent date of entry in a medical database.
I'm looking to pull the last location a patient was seen at and the doctor at that visit, which I will pass into another command later in my query. I will save those as #last_location and #doctor_name respectively. I am using #last_date to try to make a select max to get the last visit only for each given patient. However, blank values are passing forward on my variables (both #doctor_name and #last_location). Can anyone help clean this up? Appreciate the help!
My code is:
SELECT #last_location = location_mstr.location_name, #last_date = Substring (CONVERT(CHAR(8), patient_encounter.billable_timestamp, 112), 5, 2) + '/' + Substring (CONVERT(CHAR(8), patient_encounter.billable_timestamp, 112), 7, 2) + '/' + Substring (CONVERT(CHAR(8), patient_encounter.billable_timestamp, 112), 3, 2), #doctor_name = provider_mstr.description
FROM
location_mstr
INNER JOIN patient_encounter ON location_mstr.location_id = patient_encounter.location_id
INNER JOIN provider_mstr ON patient_encounter.rendering_provider_id = provider_mstr.provider_id
Where (patient_encounter.person_id = #person_id) and (#last_date = MAX(#last_date))
I also tried changing the last statement based on some threads here to:
Where #last_date = (SELECT MAX(#last_date) FROM patient_encounter WHERE patient_encounter.person_id = #person_id)

Answer 1

In the where clause of your original query, it looks like you are trying to find the patient_encounter record with the most recent billable_timestamp. The way you are referencing the #last_date variable in the where clause isn't going to do anything.
Instead, you can just get the TOP 1 most recent row and ORDER BY billable_timestamp DESC to get what you're looking for.
I also changed the string expression in your select list that sets #last_date to use the SQL CONVERT function. You can see a list of date formats that CONVERT can do here: http://msdn.microsoft.com/en-us/library/ms187928.aspx
SELECT TOP 1
#last_location = location_mstr.location_name
,#last_date = CONVERT(VARCHAR, patient_encounter.billable_timestamp, 1)
,#doctor_name = provider_mstr.description
FROM location_mstr
INNER JOIN patient_encounter
ON location_mstr.location_id = patient_encounter.location_id
INNER JOIN provider_mstr
ON patient_encounter.rendering_provider_id = provider_mstr.provider_id
WHERE patient_encounter.person_id = #person_id
ORDER BY patient_encounter.billable_timestamp DESC