What if the same id is in two files?

Those rows merge into one. A newer stamp column wins when it parses as a date. Otherwise Last or First applies. Error on conflict stops the merge.

Does First file wins ignore dates?

No. A newer last_updated (or updated_at, timestamp, and similar) still wins. First or Last is the tie-break when stamps are missing or equal.

What about empty cells?

If the winning row is blank in a field and the other file has a value, we keep the non-empty value.

Can columns differ between files?

Yes. Extra columns are kept. Missing cells are empty.

Merge CSV

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About Merge CSV

Add at least two CSV files with headers (15MB each). Key column defaults to id (any case), otherwise the first column. On conflict is Last file wins (default), First file wins, or Error on conflict. Preview is one row per distinct key, first 20 rows. Download CSV and Copy. UTF-8 BOM (Excel on Windows) defaults on. Download filename is merged.csv. Clear removes the batch.

If a stamp column exists (last_updated, updated_at, timestamp, modified, and similar names) and at least one side parses as a date, the newer date wins even when you picked First file wins. Ties then follow Last or First. Error on conflict stops at the first duplicate key and does not look at dates. Empty key values are appended, not merged. Empty cells fill from the other file when the winning row is blank.

How to use Merge CSV

1Add at least two CSV files. Each needs a header row. Use Add more CSV files for extra inputs.
2Pick Key column and On conflict. Default key is id. Last file wins, First file wins, or Error on conflict.
3Preview. One row per distinct key. First 20 rows.
4Download CSV. UTF-8, optional BOM for Excel on Windows. Copy is the same text.

Keys and conflicts

You cannot pick the timestamp column; the first matching header in that name list is used. Extra columns from any file stay. Missing cells are empty. Need two files or you see Add a second CSV to merge.

Limits and privacy

The merge runs in your browser. Files are not uploaded. 15MB per file. No account. You download a new CSV. The files you picked are not changed.

FAQs

What if the same id is in two files?: Those rows merge into one. A newer stamp column wins when it parses as a date. Otherwise Last or First applies. Error on conflict stops the merge.
Does First file wins ignore dates?: No. A newer last_updated (or updated_at, timestamp, and similar) still wins. First or Last is the tie-break when stamps are missing or equal.
What about empty cells?: If the winning row is blank in a field and the other file has a value, we keep the non-empty value.
Can columns differ between files?: Yes. Extra columns are kept. Missing cells are empty.
How large can each merge file be?: 15MB per file.