Join Multiplication

Equality Chooses Matches

Equal non-NULL keys become output rows.

highlighted = computed this step

Equality chooses matches

A candidate pair becomes output only when both keys are equal and not NULL.

equal non-NULL keys\text{equal non-NULL keys}

Read matched pairs

The join has 3 matching pairs and therefore 3 output rows.

matches=3\text{matches}=3

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.

Equality chooses matches: left tablecustomer_idname1Ada2Ben3CyNULLNoKey Equality chooses matches: 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

Matched flag

The candidate table marks each pair yes or no before rows are emitted.

matched=yes\text{matched=yes}

Summary

One matched pair creates one output row.

one pair, one row\text{one pair, one row}