Full Outer Joins

Output Row Classification

Rows are classified for the teaching render.

highlighted = computed this step

Output row classification

Each output row is classified as matched, left_only, or right_only in this teaching render.

row classes\text{row classes}

Read classes

The output has 7 rows: 2 left-only and 2 right-only rows plus the matched rows.

output rows=7\text{output rows}=7

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.

Output row classification: left tableemp_idnamedept_id1Ada102Ben203Cy304DeeNULL Output row classification: 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

Classification is visual

The class column is a renderer aid, not an SQL output column claim.

visual class aid\text{visual class aid}

Summary

The classification table makes the full outer join boundary visible.

visible boundary\text{visible boundary}