Add a consecutive number at the end of a text in attribute table

Question

I want the first column (ID_1) here (see image) to have numbers at the end of the text.

For example:

Column 1: GI_01, GI_02, GI_03 and so on. I used concat expression but it doesn't work at all as you can see in this image. I also tried to export it to CSV to auto-adjust the column and imported it to QGIS (as points ofcourse). When i tried to merge or join these column attribute table, it comes up with a NULL value or nothing at all.

Answer 1

Use @row_number with case:

concat('GI_', case when length(@row_number)<2 then '0' else '' end, @row_number-1)

Answer 2

Try this expression 'GI_' + lpad($id, 2, 0)