Join Multiplication

One Left Row Can Repeat

One left row can appear once per right-side match.

highlighted = computed this step

One left row can repeat

If one left row matches two right rows, that left row appears twice in output.

one to many\text{one to many}

Read Ada's matches

The first left row has 2 matches, and the join emits 3 rows total.

first matches=2\text{first matches}=2

Inner-join examples are tiny finite-table transforms; SQL dialect completeness, type coercion, optimizer behavior, execution cost, indexing, and database-product claims are out of scope.

One left row can repeat: left tablecustomer_idname1Ada2Ben3CyNULLNoKey One left row can repeat: right tablecustomer_idorder_iditem1101book1102pen2103bag4104lampNULL105mystery candidate row pairsleftSourcerightSourceleftKeyrightKeymatched00[1][1]yes01[1][1]yes02[1][2]no03[1][4]no04[1][NULL]no10[2][1]no11[2][1]no12[2][2]yes13[2][4]no14[2][NULL]no20[3][1]no21[3][1]no22[3][2]no23[3][4]no24[3][NULL]no30[NULL][1]no31[NULL][1]no32[NULL][2]no33[NULL][4]no34[NULL][NULL]no inner join rowsleft.customer_idleft.nameright.customer_idright.order_idright.item1Ada1101book1Ada1102pen2Ben2103bag inner join factsfactvalueleftRowCount4rightRowCount5candidatePairCount20matchingPairCount3unmatchedLeftRows2unmatchedRightRows2joinRowCount3keyColumnCount1nullKeyMatches0rowOrderleft_source_then_right_source

No automatic collapse

The output keeps both matching order rows.

both matches stay\text{both matches stay}

Summary

This is join multiplication: matched pairs, not left rows, drive row count.

pair count drives output\text{pair count drives output}