Semi Joins

Kept-Left Output

Rows with a match are kept once.

highlighted = computed this step

Kept-left output

A semi join keeps the left row when the exists flag is yes.

keep matching left rows\text{keep matching left rows}

Read output

The output keeps 3 left rows and shows only 3 left columns.

kept rows=3\text{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.

Kept-left output: left tableemp_idnamedept_id1Ada102Ben203Cy204Dee305EliNULL Kept-left output: 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

No right columns

The right table is used for the existence check, not copied into the output.

no right output columns\text{no right output columns}

Summary

The kept rows stay in left source order.

left source order\text{left source order}