Semi Joins

Left and Right Inputs

A semi join starts with two input tables.

highlighted = computed this step

Left and right inputs

A semi join starts with a left table and a right table.

left input and right input\text{left input and right input}

Read both tables

The left table has 5 rows; the right table has 4 rows.

left rows=5\text{left rows}=5

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.

Left and right inputs: left tableemp_idnamedept_id1Ada102Ben203Cy204Dee305EliNULL Left and right inputs: 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

Left rows are the output shape

A semi join only outputs columns from the left table.

left columns only\text{left columns only}

Summary

The right table is used to answer whether a match exists.

right side checks existence\text{right side checks existence}