LEFT JOIN Filters

ON Finds Right Matches

The match condition attaches right rows before WHERE.

highlighted = computed this step

ON finds matches

The ON-style key match decides which right rows attach to each left row.

match condition\text{match condition}

Read match rows

The compiler finds 3 matching right-side rows and 3 unmatched left rows.

match pairs=3\text{match pairs}=3

Outer-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.

ON finds right-side matches: left tablecidname1Ada2Bea3Cal4DeeNULLNoKey ON finds right-side matches: right tablecidorder_idstate1101paid1102open3103paid5104paidNULL105ghost match pairsleftSourceleftKeyrightSourcesmatchCount01[0, 1]212[]023[2]134[]04NULL[]0 left outer join rowsleft.cidleft.nameright.cidright.order_idright.state1Ada1101paid1Ada1102open2BeaNULLNULLNULL3Cal3103paid4DeeNULLNULLNULLNULLNoKeyNULLNULLNULL post-join filter rowsjoinSourcekeptvalues0yes[1, Ada, 1, 101, paid]1yes[1, Ada, 1, 102, open]2yes[2, Bea, NULL, NULL, NULL]3yes[3, Cal, 3, 103, paid]4yes[4, Dee, NULL, NULL, NULL]5yes[NULL, NoKey, NULL, NULL, NULL] outer join factsfactvalueleftRowCount5rightRowCount5matchPairCount3unmatchedLeftRows3paddedRows3joinRowCount6filteredRowCount6whereAppliednonullKeyLeftRows1nullKeyMatches0rowOrderleft_source_then_right_source

Match before padding

The match table is decided before NULL padding appears.

matches first\text{matches first}

Summary

ON controls matching; it is not the same stage as WHERE after the join.

match stage\text{match stage}