Inner Joins

Duplicate Matches

Multiple matching pairs produce multiple rows.

highlighted = computed this step

Duplicate matches

If one left row matches multiple right rows, each match becomes an output row.

one match one row\text{one match one row}

Read one-to-many output

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

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.

Duplicate matches: left tablesidname1Ada2Bea3CalNULLNoKey Duplicate matches: 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

No dedup step

The compiler does not collapse duplicate match rows.

bag-style output\text{bag-style output}

Summary

Inner joins preserve multiple matching pairs.

duplicates visible\text{duplicates visible}