Update every row with a value from another column from same table

Question

I have the following example of table content

+ Column1 | Column2 | Column3 +
+---------|---------|---------+
+   1     | val.txt |         +
+   2     | test.xls|         +
+   3     | abc.dwg |         +
+   4     | y.txt   |         +
+---------|---------|---------+

I want to update Column3 with information from Column2, which means I want to (in this case) extract the extension from Column2 and put it in Column3.

I want the result to look like

+ Column1 | Column2 | Column3 +
+---------|---------|---------+
+   1     | val.txt |   .txt  +
+   2     | test.xls|   .xls  +
+   3     | abc.dwg |   .dwg  +
+   4     | y.txt   |   .txt  +
+---------|---------|---------+

How to do that with an UPDATE statement?

I know how to extract the extension:

SUBSTRING(Column2, LEN(Column2)-3, LEN(Column2)) AS Extension

Answer 1

How about this:

UPDATE dbo.YourTable
SET Column3 = SUBSTRING(Column2, LEN(Column2)-3, LEN(Column2)) 

If needed, you can also include a WHERE clause to limit the rows being updated, e.g.:

UPDATE dbo.YourTable
SET Column3 = SUBSTRING(Column2, LEN(Column2)-3, LEN(Column2)) 
WHERE Column3 IS NULL

or something like that.