Inner Joins

NULL Join Keys Do Not Match

NULL keys do not match, even against NULL.

highlighted = computed this step

NULL join keys do not match

A NULL join key is not equal to another NULL join key in this scoped join model.

NULL does not match NULL\text{NULL does not match NULL}

Read NULL-key facts

NULL-key matches are 0.

NULL matches=0\text{NULL matches}=0

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.

NULL join keys do not match: left tablesidname1Ada2Bea3CalNULLNoKey NULL join keys do not match: 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

No padded rescue

Because this is an inner join, the NULL-key left row is dropped.

no match, no row\text{no match, no row}

Summary

NULL-key nonmatching and unmatched-row dropping are separate checks.

no NULL equality\text{no NULL equality}