Join Multiplication

Qualified Output Columns

Qualified column names keep both sides readable.

highlighted = computed this step

Qualified output columns

The output keeps left and right columns qualified so repeated names stay readable.

left dot column, right dot column\text{left dot column, right dot column}

Read output width

The joined output has 5 qualified columns across 3 rows.

columns=5\text{columns}=5

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.

Qualified output columns: left tablecustomer_idname1Ada2Ben3CyNULLNoKey Qualified output columns: 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

Read both customer ids

Both customer_id columns remain visible in this teaching output.

qualified names\text{qualified names}

Summary

Qualification is a display convention here, not a product SQL claim.

visible columns\text{visible columns}