Inner Joins

Base Left and Right Tables

An inner join starts with two visible input tables.

highlighted = computed this step

Base left and right tables

An inner join starts with a left table and a right table.

two input tables\text{two input tables}

Read the inputs

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

left rows=4\text{left rows}=4

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.

Base left and right tables: left tablesidname1Ada2Bea3CalNULLNoKey Base left and right tables: 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

Keep both sides visible

The visual keeps both input tables visible before any output row is trusted.

inputs before output\text{inputs before output}

Summary

Inner joins are easiest to read when the input rows stay in view.

visible inputs\text{visible inputs}