Quick question, why not restore the db from a backup and then drop the unwanted tables?
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Looks like the grouping is on the date, dense rank should do the trick
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Can you please post the DDL (create table) script, sample data as an insert statement and the desired resuls
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This can get quite messy very quickly, suggest you first do the checks and then if all is clear, do the insert at the end. This way you can display...

Quick suggestion, catch the unwanted values in a NULLIF function, no need to use a CASE statement here.
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DECLARE @exchangeRate FLOAT = 0;

;WITH SAMPLE_DATA AS

Can you please post some sample data as an insert statement?
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Quick thought, I don't think you are going to get far tackling performance issues by analyzing the transaction logs, probably better off with something like Solar Winds DPA, Idera or...

Are you exporting the data from a table?
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Quick thought, use a row number or an id number, divide by the max row size and wrap this in either a view or a inline table valued function. Then...

As Thom said, no need for a case statement, try_cast and try_convert simply return null if the value cannot be converted
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Here is a try_convert example

What is the output of this query?
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SELECT
  mf.database_id
 ,mf.name
 ,mf.Physical_Name
  FROM Master.sys.master_files mf
   INNER JOIN Master.sys.databases db ON db.database_id =...