Join Multiplication

Two Input Tables

Join multiplication starts with two tables.

highlighted = computed this step

Two input tables

Join multiplication starts with a left table and a right table.

two inputs\text{two inputs}

Show both sides

The left table has 4 rows, and the right table has 5 rows.

left rows=4\text{left rows}=4

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.

Two input tables: left tablecustomer_idname1Ada2Ben3CyNULLNoKey Two input tables: 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

Keep keys visible

Both tables expose customer_id so the pair check is visible.

key columns visible\text{key columns visible}

Summary

No output row is trusted until the candidate pairs are checked.

inputs first\text{inputs first}