LEFT JOIN Filters

Left and Right Tables

LEFT JOIN filters start with two visible tables.

highlighted = computed this step

Left and right tables

A LEFT JOIN starts with a left table and a right table.

two input tables\text{two input tables}

Read the inputs

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

left rows=5\text{left rows}=5

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.

Left and right tables: left tablecidname1Ada2Bea3Cal4DeeNULLNoKey Left and right tables: 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

Keep both sides visible

The visual keeps both tables visible before any filter is trusted.

inputs first\text{inputs first}

Summary

The rest of the book changes what happens after matching.

matching then filtering\text{matching then filtering}