Merge CSV & Excel Files
Combine several CSV or Excel files into one: stack them (january.csv + february.csv + march.csv → one file with the whole quarter) or join them by a shared column, like a database JOIN.
Stacking rows vs. joining by column
Stacking is for when several files share the same columns and you want to combine all their rows into one — monthly sales exports, for example. Joining by column (a JOIN) is for when you have two different files that share an identifier (like customer_id) and want to combine their columns into a single table — matching a customer list against their orders, for example.
Inner, left and full join
Left join keeps every row from file A, with B's columns filled in only where there's a match. Inner join keeps only rows that match in both files. Full join also adds B's rows that have no match in A. There's no separate right join since it's equivalent to a left join with the files swapped.
What happens if a column is missing
When stacking, if one file lacks a column the others have, that column is filled in blank for that file and a warning is shown — the whole file is never dropped just for a missing column.