CSV left join

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

Left join two CSV documents on exact named key columns.

Use when you need to: csv left join · left join csv documents · left join csv on named keys.

Supported

• csv left join
• left join csv documents
• left join csv on named keys

Not supported

• inner join
• right join
• full outer join
• fuzzy keys
• numeric keys
• read files
• fetch urls

Behavior

• left and right are complete CSV documents whose first records are headers.
• keys names unique headers present in both documents; join uses exact string tuples with no coercion.
• Output columns are the keys in listed order, then remaining left columns in left header order, then remaining right columns in right header order.
• Non-key header collisions are renamed with left_suffix and right_suffix (defaults _left and _right); remaining collisions throw.
• Every left data row appears; matching is a cartesian product of right rows with the same key, in left then right order.
• Left rows with no match emit empty strings for right-only columns; unmatched_left_rows counts those left rows.
• Empty documents without headers are rejected; header-only inputs yield the computed header and zero data rows.
• A join key may not appear on more than 64 data rows on either side.
• The first output header name may not begin with U+FEFF because it is ambiguous with a CSV byte-order mark.

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)
• unmatched_left_rows (integer, required)

Limits

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

Example

Request input:

{
  "left": "id,name\n1,Ada\n2,Lin\n3,Grace\n",
  "right": "id,city\n1,London\n1,Paris\n4,Tokyo\n",
  "keys": [
    "id"
  ]
}

Response:

{
  "result": {
    "csv": "id,name,city\n1,Ada,London\n1,Ada,Paris\n2,Lin,\n3,Grace,\n",
    "rows_out": 4,
    "unmatched_left_rows": 2
  }
}

How to call it

MCP

Connect https://computefirst.net/mcp (setup), then call execute with:

{
  "id": "csv-left-join",
  "version": "1.0.0",
  "input": {
    "left": "id,name\n1,Ada\n2,Lin\n3,Grace\n",
    "right": "id,city\n1,London\n1,Paris\n4,Tokyo\n",
    "keys": [
      "id"
    ]
  }
}

HTTP (no key)

curl -X POST https://computefirst.net/v1/tools/csv-left-join/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"left":"id,name\n1,Ada\n2,Lin\n3,Grace\n","right":"id,city\n1,London\n1,Paris\n4,Tokyo\n","keys":["id"]}'

The machine-readable contract is at /v1/tools/csv-left-join/versions/1.0.0.

CLI

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

Get the client at /clients/cli/.

Related tools

• CSV inner join: Inner join two CSV documents on exact named key columns.
• CSV anti join: Keep left CSV rows whose exact key tuples do not appear in the right document.
• CSV concat: Concatenate CSV documents that share identical headers, preserving document and row order.
• CSV dedupe: Remove duplicate CSV data rows while preserving the first occurrence.
• CSV merge columns: Join named CSV columns with an exact separator into one column.
• 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.