Semi Joins

Candidate Pairs

Candidate pairs show every left-right comparison.

highlighted = computed this step

Candidate pairs

The compiler compares each left row with each right row.

candidate pair grid\text{candidate pair grid}

Read pair count

The candidate grid has 20 left-right pairs.

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

Semi-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 pairs: left tableemp_idnamedept_id1Ada102Ben203Cy204Dee305EliNULL Candidate pairs: right tabledept_iddept_name10ops20eng20platformNULLunknown candidate pairsleftSourcerightSourceleftKeyrightKeymatched00[10][10]yes01[10][20]no02[10][20]no03[10][NULL]no10[20][10]no11[20][20]yes12[20][20]yes13[20][NULL]no20[20][10]no21[20][20]yes22[20][20]yes23[20][NULL]no30[30][10]no31[30][20]no32[30][20]no33[30][NULL]no40[NULL][10]no41[NULL][20]no42[NULL][20]no43[NULL][NULL]no existence per left rowleftSourceleftKeymatchingRightSourcesexistskept0[10][0]yesyes1[20][1, 2]yesyes2[20][1, 2]yesyes3[30][]nono4[NULL][]nono semi join kept-left rowsemp_idnamedept_id1Ada102Ben203Cy20 semi join factsfactvalueleftRowCount5rightRowCount4candidatePairCount20matchingPairCount5leftRowsWithMatch3keptLeftRows3droppedLeftRows2duplicateRightMatches2nullKeyCandidatePairs8modesemirowOrderleft_source_order_once

Pairs are not output rows

The pair grid explains matching; it is not the semi-join output.

pairs only explain\text{pairs only explain}

Summary

A semi join looks for existence inside the pair grid.

existence next\text{existence next}