Inner Joins

Matching Key Pairs

Only equal non-NULL keys become output rows.

highlighted = computed this step

Matching key pairs

A pair becomes output only when the join keys are equal and not NULL.

key equality\text{key equality}

Read matches

The compiler finds 3 matching pairs and produces 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.

Matching key pairs: left tablesidname1Ada2Bea3CalNULLNoKey Matching key 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

Matched flag

The candidate table marks each pair yes or no.

matched=yes\text{matched=yes}

Summary

An inner join keeps only the yes pairs.

yes pairs kept\text{yes pairs kept}