Semi Joins

Honest by Construction

Expected semi-join facts are assertions only.

highlighted = computed this step

Honest by construction

Expected semi-join facts are assertions only.

compiler is source of truth\text{compiler is source of truth}

Render checked rows

The guarded semi join keeps 3 left rows after recomputation.

checked kept rows=3\text{checked kept rows}=3

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.

Honest by construction: left tableemp_idnamedept_id1Ada102Ben203Cy204Dee305EliNULL Honest by construction: 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

Try to tamper

Wrong expected facts, derived sidecars, duplicate columns, bools, floats, missing keys, and unsupported modes fail closed.

tamper rejected\text{tamper rejected}

Summary

This book shows exact finite semi and anti-semi joins only.

scoped semi join model\text{scoped semi join model}