确定nvarchar列中的varchar内容

Question

I have a bunch of NVARCHAR columns which I suspect contain perfectly storable data in VARCHAR columns. However I can't just go and change the columns' type into VARCHAR and hope for the best, I need to do some sort of check.

I want to do the conversion because the data is static (it won't change in the future) and the columns are indexed and would benefit from a smaller (varchar) index compared to the actual (nvarchar) index.

If I simply say

ALTER TABLE TableName ALTER COLUMN columnName VARCHAR(200)

then I won't get an error or a warning. Unicode data will be truncated/lost.

How do I check?

Answer 1

Why not cast there and back to see what data gets lost?

This assumes column is nvarchar(200) to start with

SELECT *
FROM TableName
WHERE columnName <> CAST(CAST(columnName AS varchar(200)) AS nvarchar(200))

Answer 2

Hmm interesting.

I'm not sure you can do this in a SQL query itself. Are you happy to do it in code? If so, you can get all the records, then loop over all the chars in the string and check. But man it's a slow way.