WHERE vs HAVING

WHERE Filters Rows

WHERE filters source rows before groups exist.

highlighted = computed this step

WHERE filters rows

WHERE decides row by row before GROUP BY exists.

WHERE sees rows\text{WHERE sees rows}

Count row decisions

The row filter keeps 4 rows and drops 2 rows before grouping.

kept rows=4\text{kept rows}=4

HAVING examples are tiny finite group-filter transforms; SQL dialect completeness, optimizer behavior, indexing, and product claims are out of scope.

WHERE filters rows first: base rowsidregionstatusqty1eastopen22eastopen33westopen14westclosed45northNULL26northopen1 WHERE before groupingsourceinputwhereTruthgroupedRole0[1, east, open, 2]TRUEgrouped1[2, east, open, 3]TRUEgrouped2[3, west, open, 1]TRUEgrouped3[4, west, closed, 4]FALSEdropped_before_grouping4[5, north, NULL, 2]UNKNOWNdropped_before_grouping5[6, north, open, 1]TRUEgrouped GROUP BY bucketskeysourceRowsrowCount[east][0, 1]2[west][2]1[north][5]1 Aggregate facts per groupkeysourceRowsaggregates[east][0, 1]{n:2, total_qty:5}[west][2]{n:1, total_qty:1}[north][5]{n:1, total_qty:1} HAVING result per groupkeypredicatetruthoutputRole[east]noneTRUEkept[west]noneTRUEkept[north]noneTRUEkept HAVING output groupsregionntotal_qtyeast25west11north11 HAVING factsfactvalueinputRowCount6whereKeptRows4whereDroppedRows2groupCount3aggregateCount2outputGroupCount3havingTrueCount3havingFalseCount0havingUnknownCount0groupOrderfirst_seen_after_wherepipelineWHERE_before_GROUP_BY_then_HAVING

Dropped means gone

A row dropped by WHERE never contributes to a later group total.

drop before group\text{drop before group}

Summary

WHERE is the row boundary.

row filter first\text{row filter first}