Group Concatenate NDJSON Files Online
Concatenate text values within each group in NDJSON files directly in your browser. Group rows by a column and join the values of another column into a single delimited string — no upload required.
How to group Concatenate NDJSON files
- Drop your file onto the upload area. The first 200 rows are shown so you can check the columns loaded correctly.
- Pick a Group by column, or leave it on "No grouping (whole file)" to join every value into a single cell.
- Choose the column to concatenate values from, then set the separator (", " by default) and the output column name ("values" by default). Type \n in the separator for a new line or \t for a tab.
- Optionally pick a column under "Order values within group by" and a sort direction, so the joined values come out in a predictable order.
- Click Concatenate Values, check the result, and download it in the same format you uploaded. The file has one row per group.
Your file is processed locally in your browser and is never uploaded. The free limit is 50 MB per file; larger files work if your device has the memory for them.
Worked example
A blog CMS exports one row per article and tag. The editor wants a single row per article with all its tags in one cell, sorted alphabetically, to paste into a content audit.
Input (NDJSON)
{"article_slug":"csv-vs-parquet","tag":"parquet","added_by":"mia"}
{"article_slug":"csv-vs-parquet","tag":"csv","added_by":"mia"}
{"article_slug":"json-basics","tag":"json","added_by":"raj"}
{"article_slug":"csv-vs-parquet","tag":"file-formats","added_by":"omar"}
{"article_slug":"json-basics","tag":"apis","added_by":"raj"}Settings
- Group by: article_slug
- Concatenate values from: tag
- Separator: ", "
- Order values within group by: tag, Ascending
- Output column name: tags
Result
| article_slug | tags |
|---|---|
| csv-vs-parquet | csv, file-formats, parquet |
| json-basics | apis, json |
Five rows become two, one per article. Only the group column and the new tags column are kept, so added_by is dropped. Tags are sorted A to Z inside each cell because tag was chosen as the ordering column. The output rows themselves are sorted by article_slug. Without an ordering column, the tags would still be joined, but their order inside each cell is not guaranteed.
Frequently Asked Questions
Which columns are kept in the output?
Only the group-by column and the new concatenated column. Every other column is dropped, because the result has one row per group. With no grouping, the output is a single row with a single column.
Are NULL values included in the joined string?
No. NULLs are skipped. If every value in a group is NULL, the joined cell for that group is NULL rather than an empty string.
Can I use a newline as the separator?
Yes. Type \n in the separator box and each value goes on its own line. \t inserts a tab. To join with a real backslash, type \\ for each backslash you want.
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