Add lookup fields without changing your original rows
Choose an exact ID in each export and append selected lookup fields. Repeated left IDs are allowed. Every nonblank lookup ID must be unique, including unused IDs. Empty and whitespace-only IDs never match.
Each input: UTF-8, 2 MiB, 20,000 data records, 100 columns, 200,000 cells including header. Result: 100 columns, 200,000 total cells and 2 MiB prospective fully quoted comma CSV, including CRLF and possible prefixes. Small inputs can amplify beyond these limits. A file selection replaces pasted text even when rejected.
Left file
Lookup file
Lookup fields to add
Fields are appended in the order added. Give each an explicit output name; colliding or blank names block merging. Remove and re-add a field to change its order.
Ready for two exports.
Merge report
Preview shows at most 100 rows. Exact JSON contains every row, raw strings, left/lookup references and match reasons. Matched empty values differ from unmatched empty values only in metadata.
Prefix mode adds an apostrophe to formula/control-leading cells, including = + - @ after whitespace and leading tab/newline. It changes values and is not a universal spreadsheet safety guarantee. Literal CSV preserves raw strings but spreadsheets may interpret formulas. JSON and preview always remain raw.
Worked example: repeated orders and empty reference values
Load the example, read both files and choose id in each. Add category as supplier_category, then color as supplier_color. The left color stays unchanged. Six orders become six rows: three matched and three unmatched, including two blank IDs and one missing ID. Two blank lookup rows are ignored; two usable keys are used and one is unused. The matched 123 reference has an empty color; its metadata still says matched. Adding a second UNUSED lookup row blocks the entire merge.
Limits and data handling
Keys remain exact strings: 00123 differs from 123; case and nonblank spaces matter. No trimming, numeric conversion, guessing, repair, import certification or business validation. Strict parser failure shows the first error and no partial counts. Parser lines are physical lines; quoted newlines can make them differ from logical records. Source references count the header as record 1.
Files are read and merged locally without uploads, page-URL input values, analytics, advertising or browser storage. A host serves pages and assets. Downloads create new files; originals remain unchanged. Clear removes displayed values and results, but saved downloads remain on your device.