How to merge two CSV files
- Add the main file, for example your orders. Every row in it is kept.
- Add the lookup file, for example a list of customers or products.
- Pick the matching columns. They are guessed from column names, and they don't need the same name in both files.
- Choose the columns to bring across and click Merge. Download the result as CSV or Excel.
Which join do I need?
| Option | Result |
|---|---|
| All main rows (left join) | Every row of the main file, with the lookup columns filled in where a match was found and left blank where not. This is what VLOOKUP and XLOOKUP do. |
| Only matching rows (inner join) | Only rows whose key exists in both files. |
| Everything (full join) | All rows from both files, matched where possible. Useful for reconciling two lists. |
Why VLOOKUP returns #N/A, and how this avoids it
Most failed lookups are caused by keys that look the same but aren't: R03 and r03 with a trailing space, or numbers stored as text. With “Ignore case and spaces” switched on, those still match. Rows that still don't match are counted and flagged in the _match column (left only or right only), so you can see them instead of hunting for #N/A.
If a key appears more than once in the lookup file, a join repeats the main row once per match, which is correct for SQL but surprising if you expect VLOOKUP. Tick First match only to keep one row per main row, exactly like VLOOKUP. The tool warns you when keys are repeated.
Need to match on several columns or do more complex joins? Load both files into SQL on CSV and write the join yourself. To spot differences between two versions of the same file, use CSV Compare.
Frequently asked questions
How do I do a VLOOKUP between two CSV files?
Add the file you want to keep as the main table and the file with the extra details as the lookup table, pick the matching columns and click Merge with “All main rows” selected. Tick “First match only” to copy VLOOKUP’s behaviour exactly.
Do the key columns need the same name?
No. You choose the matching column in each file separately. The tool suggests columns with the same name, or ID-like columns, to start with.
Can I merge Excel files?
Yes. Both sides accept .xlsx, .xls and .ods files (pick the sheet), as well as CSV, TSV and JSON, and you can mix formats.
How do I find rows that did not match?
Leave “Add a _match column” ticked. Unmatched rows are marked “left only” (only in the main file) or “right only” (only in the lookup file, with a full join), tinted in the table, and counted in the summary. Type “left only” in the filter to list them.
Are my files uploaded?
No. Both files are read and joined in your browser.