Append two CSVs or database-style join them on a shared key column.
output
About this tool
CSV Merge / Join is a free, in-browser tool that combines two CSVs — either by appending their rows or by performing a database-style join on a shared key column. Both files stay on your device; nothing is uploaded.
Append mode stacks the two files and unions their columns, filling blanks where a file lacks a column. Join mode matches rows on the key you name (by header or index) and supports inner, left, right, and full-outer joins, so it works like a spreadsheet VLOOKUP but with every join type. Unmatched keys are filled with blank cells according to the join you pick.
Because the hash-join runs entirely locally it is a private, offline-friendly way to reconcile exports, enrich one table with another, or concatenate related files without a spreadsheet or database.
Frequently asked questions
What do the join modes do?
Inner keeps only keys present in both files; left keeps every row of file A (with blanks where B has no match); right keeps every row of file B; and full-outer keeps all keys from either side. Append instead stacks all rows and unions the columns.
How do I specify the key column?
Enter the key by header name (like id) or by zero-based index (like 0). The key from file A is kept once, and file B's remaining columns are appended to the right of the matched row.
What happens with duplicate keys?
If a key appears multiple times in either file, the join produces every matching combination (a Cartesian expansion for that key), mirroring how SQL joins behave with non-unique keys.
What CSV conventions apply?
Both files must share the same delimiter, quoted fields (inner quotes doubled) are parsed correctly, and the header setting applies to both files at once — tick it when the first row of each file names its columns.