# CSV inner join

`csv-inner-join` · version 1.0.0 · CSV & tables · free, no key needed

Inner join two CSV documents on exact named key columns.

**Use when you need to: csv inner join · inner join csv · join csv documents on keys.**

## Supported

- csv inner join
- inner join csv
- join csv documents on keys

## Not supported

- outer join
- left join
- fuzzy keys
- numeric keys
- read files
- fetch urls

## Behavior

- left and right are complete CSV documents whose first records are headers.
- Empty documents without headers are rejected; header-only documents yield a header-only join.
- keys names unique headers present on both sides; join equality uses those exact cell strings with no coercion.
- Output columns are the keys in listed order, then leftover left columns in left header order, then leftover right columns in right header order.
- Non-key headers that appear on both sides are renamed with left_suffix and right_suffix; keys are never suffixed. Remaining collisions throw.
- Each left row is concatenated with every matching right row. Output follows left row order, then matching right row order. Unmatched left rows and right-only keys are omitted.
- A key tuple with more than 64 rows on either side is rejected even when that key would not appear in the output.
- Output data rows cannot exceed the CSV row limit. The first output header must not begin with U+FEFF.

## Input

- `left` (string, required): max length 256000
- `right` (string, required): max length 256000
- `keys` (array of string, required): min items 1; max items 32; each min length 1
- `left_suffix` (string, optional): min length 1; max length 32; default `"_left"`
- `right_suffix` (string, optional): min length 1; max length 32; default `"_right"`

## Output

- `csv` (string, required)
- `rows_out` (integer, required)

## Limits

- max input bytes: 256000
- max data rows: 5000
- max columns: 256
- max header name bytes: 256
- max keys: 32
- max duplicate rows per key: 64
- max suffix chars: 32
- max output bytes: 256000

## Example

Request input:

```json
{
  "left": "id,name,city\n1,Ada,London\n2,Lin,Paris\n3,Zoe,Rome\n",
  "right": "id,dept,city\n1,Math,Cambridge\n1,CS,Oxford\n2,Physics,Paris\n4,Art,Berlin\n",
  "keys": [
    "id"
  ]
}
```

Response:

```json
{
  "result": {
    "csv": "id,name,city_left,dept,city_right\n1,Ada,London,Math,Cambridge\n1,Ada,London,CS,Oxford\n2,Lin,Paris,Physics,Paris\n",
    "rows_out": 3
  }
}
```

## How to call it

### MCP

Connect `https://computefirst.net/mcp` ([setup](/docs#connect)), then call `execute` with:

```json
{
  "id": "csv-inner-join",
  "version": "1.0.0",
  "input": {
    "left": "id,name,city\n1,Ada,London\n2,Lin,Paris\n3,Zoe,Rome\n",
    "right": "id,dept,city\n1,Math,Cambridge\n1,CS,Oxford\n2,Physics,Paris\n4,Art,Berlin\n",
    "keys": [
      "id"
    ]
  }
}
```

### HTTP (no key)

```sh
curl -X POST https://computefirst.net/v1/tools/csv-inner-join/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"left":"id,name,city\n1,Ada,London\n2,Lin,Paris\n3,Zoe,Rome\n","right":"id,dept,city\n1,Math,Cambridge\n1,CS,Oxford\n2,Physics,Paris\n4,Art,Berlin\n","keys":["id"]}'
```

The machine-readable contract is at [/v1/tools/csv-inner-join/versions/1.0.0](/v1/tools/csv-inner-join/versions/1.0.0).

### CLI

```sh
node cli.mjs run csv-inner-join 1.0.0 --input input.json --base-url https://computefirst.net
```

Get the client at [/clients/cli/](/clients/cli/).

## Related tools

- [CSV left join](/tools/csv-left-join): Left join two CSV documents on exact named key columns.
- [CSV anti join](/tools/csv-anti-join): Keep left CSV rows whose exact key tuples do not appear in the right document.
- [CSV concat](/tools/csv-concat): Concatenate CSV documents that share identical headers, preserving document and row order.
- [CSV merge columns](/tools/csv-merge-columns): Join named CSV columns with an exact separator into one column.
- [CSV reconcile](/tools/csv-reconcile): Reconcile two CSV documents by key columns, reporting full population totals and bounded samples for added, removed, changed, invalid-key, and ambiguous rows.
- [CSV row diff](/tools/csv-row-diff): Compare two CSV documents with identical headers by unique key tuples and report added, removed, and changed rows.