Inner Joins

Candidate Row Pairs

Every left row can be compared with every right row.

highlighted = computed this step

Candidate row pairs

Before matching keys, every left row can be compared with every right row.

candidate pairs\text{candidate pairs}

Count candidates

The compiler checks 20 candidate row pairs.

candidate pairs=20\text{candidate pairs}=20

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.

Candidate row pairs: left tablesidname1Ada2Bea3CalNULLNoKey Candidate row pairs: right tablesidscore1A1A23C4DNULLGhost candidate row pairsleftSourcerightSourceleftKeyrightKeymatched00[1][1]yes01[1][1]yes02[1][3]no03[1][4]no04[1][NULL]no10[2][1]no11[2][1]no12[2][3]no13[2][4]no14[2][NULL]no20[3][1]no21[3][1]no22[3][3]yes23[3][4]no24[3][NULL]no30[NULL][1]no31[NULL][1]no32[NULL][3]no33[NULL][4]no34[NULL][NULL]no inner join rowsleft.sidleft.nameright.sidright.score1Ada1A1Ada1A23Cal3C inner join factsfactvalueleftRowCount4rightRowCount5candidatePairCount20matchingPairCount3unmatchedLeftRows2unmatchedRightRows2joinRowCount3keyColumnCount1nullKeyMatches0rowOrderleft_source_then_right_source

Read source order

Candidates are listed by left source row, then right source row.

left source then right source\text{left source then right source}

Summary

Candidate pairs are the rows that might match.

maybe matches\text{maybe matches}