Join Row-Count Checker
Predict row multiplication and unmatched output before an equality join using aggregate key counts.
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Predict row multiplication before an equality join using counts per key. Enter fictional labels for aggregate groups only, never patient identifiers or actual record-level data. Defaults are fictional. Each line is label,left count,right count. Include unmatched groups with a zero on the missing side.
Calculation results
Inner join: 8 rows
Left join: 9 rows (from 4 left rows). Full join: 10 rows.
1 key groups have duplicates on both sides. Every matching left row combines with every matching right row.
| Key group | Left | Right | Inner output |
|---|---|---|---|
| Example-A | 1 | 2 | 2 |
| Example-B | 2 | 3 | 6 |
| Example-C | 1 | 0 | 0 |
| Example-D | 0 | 1 | 0 |
Matching rules matter
For each exact nonmissing key, inner output = left count × right count. A left join retains unmatched left rows; a full join also retains unmatched right rows. Labels are case-sensitive and trimmed. Count tables must describe all intended keys; no header row is accepted. Missing/null keys, composite-key construction, case normalization, filters and deduplication must be resolved in the actual software. This calculator predicts counts; it does not merge files, check identities or decide whether duplication is justified.
dplyr documentation: join relationships and unmatched keys · Read the practical guide · Download blank worksheet (TXT)