CSV Merge Tool
Join two CSV datasets on a shared key with inner, left, right or full join semantics, see which rows matched, and export the combined result without a database or a spreadsheet.
Last reviewed by the Radiatus Cloud team
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The lookup everyone does in a spreadsheet
Combining two exports on a shared identifier is one of the most common data tasks there is: orders against customers, users against subscription records, campaign data against conversions. Most people reach for a lookup formula, drag it down a few thousand rows, and then spend longer than they expected working out why some cells came back empty. The empty cells are the interesting part, and a formula hides them.
Join type is the decision that matters
An inner join keeps only rows that matched on both sides, which is right when you need complete records. A left join keeps every row from the first dataset and fills blanks where there was no match, which is right when the first file is the population you care about and you want to see the gaps. A full join keeps everything from both. Choosing wrongly silently drops rows, and dropped rows in a reconciliation are exactly the ones that mattered.
Reporting what did not match
This tool performs the join and then tells you how many rows matched, how many keys were unmatched on each side, and whether any key appeared more than once, which is what turns a join into an unexpected row multiplication. Those three numbers catch most merge errors before the result is used, and none of them are visible in a spreadsheet lookup.
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Frequently Asked Questions
What is the difference between the join types?
Inner keeps only matched rows. Left keeps every row from the first dataset. Right keeps every row from the second. Full keeps everything from both, padding with blanks. Row counts for each are shown so you can see the effect before exporting.
Why did my result have more rows than either input?
Because a key appears more than once on one side, so each match multiplies. The tool reports duplicate keys explicitly, since this is the most common surprise in a merge and it is easy to miss in a spreadsheet.
Are key comparisons case sensitive?
By default yes, since identifiers usually are. There is an option to trim whitespace and compare case insensitively, which is what you want when joining on an email address or a name.
What happens to columns with the same name in both files?
The second file’s column is suffixed so both values survive. Silently overwriting one with the other would lose data without any indication that it happened.
How large a file can it handle?
It works comfortably with tens of thousands of rows in a browser tab. Beyond that, a database or a command line tool is a better fit, mostly because holding both datasets and the result in memory becomes the limit.
Privacy & Security
Everything runs in your browser; nothing is uploaded.
How to Use
Paste two CSV datasets, choose the key column in each, pick a join type and export the merged result.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
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