How to combine data by key?

Question

Data processing

There is a simple table consisting of two columns, but it has about two million rows.
The table looks like

01 | a245
02 | a768
03 | b458
03 | g234
04 | t797
04 | u562
04 | o577

Need this format

01 | a245
02 | a768
03 | b458 | g234
04 | t797 | u562 | o577

I don't know how to formulate the task correctly for myself... Collapse the data?
Source data in txt format, separator "semicolon"
In which direction to dig? excel?

Answer 1

it is obvious that the numbers are redundant. We copy everything to Excel and demolish them together with the first column.
Next, remove the paragraph mark. We get a long kosher string with separators for 2 million records.
Further, we notice that each number with a separator takes 6 bytes, so after each N * 6 insert a paragraph sign. Here my imagination broke down and I think it will have to be done with a script.
The result is driven into Excel in the 2nd column, in the first we add the line numbers.