Full Outer Joins

Duplicate Keys Multiply Matches

Duplicate matching keys still multiply output rows.

highlighted = computed this step

Duplicate keys multiply matches

If one left key matches more than one right row, each matching pair becomes a row.

duplicate match rows\text{duplicate match rows}

Read duplicate matches

The duplicate right-side department creates 1 extra matched row inside 3 total matches.

extra matches=1\text{extra matches}=1

Full-outer-join examples are tiny finite-table transforms; SQL dialect completeness, type coercion, optimizer behavior, execution cost, indexing, output ordering guarantees, and database-product claims are out of scope.

Duplicate keys multiply matches: left tableemp_idnamedept_id1Ada102Ben203Cy304DeeNULL Duplicate keys multiply matches: right tabledept_iddept_name10ops20eng20platform40legalNULLunknown candidate key matchesleftSourcerightSourceleftKeyrightKeymatched001010yes011020no021020no031040no0410NULLno102010no112020yes122020yes132040no1420NULLno203010no213020no223020no233040no2430NULLno30NULL10no31NULL20no32NULL20no33NULL40no34NULLNULLno FULL OUTER JOIN output rowsleft.emp_idleft.nameleft.dept_idright.dept_idright.dept_name1Ada1010ops2Ben2020eng2Ben2020platform3Cy30NULLNULL4DeeNULLNULLNULLNULLNULLNULL40legalNULLNULLNULLNULLunknown output row classificationclassleftSourcerightSourcerowmatched00[1, Ada, 10, 10, ops]matched11[2, Ben, 20, 20, eng]matched12[2, Ben, 20, 20, platform]left_only2NULL[3, Cy, 30, NULL, NULL]left_only3NULL[4, Dee, NULL, NULL, NULL]right_onlyNULL3[NULL, NULL, NULL, 40, legal]right_onlyNULL4[NULL, NULL, NULL, NULL, unknown] FULL OUTER JOIN factsfactvalueleftRowCount4rightRowCount5candidatePairCount20matchingPairCount3leftOnlyRowCount2rightOnlyRowCount2outputRowCount7duplicateMatchRows1nullKeyCandidatePairs8rowOrdermatched_left_source_then_left_only_then_right_only

No dedup

FULL OUTER JOIN does not deduplicate matching pairs in this bag-style model.

no implicit DISTINCT\text{no implicit DISTINCT}

Summary

The full join keeps join multiplication for matching keys.

matches still multiply\text{matches still multiply}