Merge CSV files

Merging combines rows from several exports into one table: twelve monthly reports, a survey collected in batches, or the transaction files a platform insists on capping at 5,000 rows each. The first row of every file is read as its header and never lands in the data.

Union mode aligns columns by name rather than by position. That is the part a plain copy and paste gets wrong, because two exports of the same report often carry the same fields in a different order, and stacking them blindly puts email addresses under the postcode heading. Union builds one complete header set, keeps each value under its own name, and leaves a blank where a file simply had no such column. Strict mode is the opposite instinct: every file must present the same headers in the same order, and the merge stops naming the file that disagrees. Reach for strict when the files are supposed to be identical and a mismatch means something has gone wrong upstream.

Header matching is exact, including capitalisation and stray spaces, so Email and email arrive as two separate columns with half the rows blank in each. When the result has more columns than you expected, that is nearly always the reason, and tidying the headers before merging is quicker than repairing the output.

Delimiters are detected per file, so a semicolon export from a European spreadsheet merges cleanly with a comma one, and quoted commas, multiline cells, and Unicode text survive. A parse failure is reported with the file name and row number rather than a generic error. Values that could be read as spreadsheet formulas are written back as text, so a merged file cannot carry an executable cell into someone else’s spreadsheet.

Rows keep their order: file by file in the order shown in the list, and row by row within each file. Turning on the source label adds a column recording which file supplied each row, which is the fastest way to trace a suspect figure back to its export. All of it runs in your browser, so customer lists and internal financial exports are never uploaded.