JOIN USING

Duplicate Key Matches Multiply

Duplicate matching keys still multiply output rows.

highlighted = computed this step

Duplicate key matches multiply

If one left key matches multiple right rows, each pair becomes an output row.

duplicate key matches\text{duplicate key matches}

Read multiplication

The duplicate key creates 2 extra match rows inside 5 total matches.

extra matches=2\text{extra matches}=2

JOIN USING examples are tiny finite-table transforms; SQL dialect completeness, type coercion, optimizer behavior, execution cost, indexing, output ordering guarantees, and database-product claims are out of scope.

Duplicate key matches multiply: left tableorder_idcustomer_idnote110new220rush320gift4NULLdraft Duplicate key matches multiply: right tablecustomer_idnamenote10Adavip20Benstandard20Bennyduplicate30CyprospectNULLUnknownmissing candidate key comparisonsleftSourcerightSourceleftKeyrightKeymatched001010yes011020no021020no031030no0410NULLno102010no112020yes122020yes132030no1420NULLno202010no212020yes222020yes232030no2420NULLno30NULL10no31NULL20no32NULL20no33NULL30no34NULLNULLno JOIN USING output rowscustomer_idleft.order_idleft.noteright.nameright.note101newAdavip202rushBenstandard202rushBennyduplicate203giftBenstandard203giftBennyduplicate ON join contrast rowsleft.order_idleft.customer_idleft.noteright.customer_idright.nameright.note110new10Adavip220rush20Benstandard220rush20Bennyduplicate320gift20Benstandard320gift20Bennyduplicate JOIN USING factsfactvalueleftRowCount4rightRowCount5candidatePairCount20matchingPairCount5matchedLeftRows3duplicateMatchRows2nullKeyCandidatePairs8usingKeyAppearsOnceyesduplicateNonKeyNames[note]outputColumnCount5onOutputColumnCount6rowOrderleft_source_then_right_source

USING does not dedup

Merging the key column does not remove duplicate matching pairs.

layout not dedup\text{layout not dedup}

Summary

JOIN USING changes output layout, not match multiplicity.

same match count\text{same match count}