WHERE vs HAVING

Groups After WHERE

Only rows that pass WHERE enter groups.

highlighted = computed this step

Groups after WHERE

Only rows that passed WHERE are placed into group buckets.

passed rows group\text{passed rows group}

Read buckets

4 kept rows form 3 groups in first-seen order.

groups=3\text{groups}=3

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

Groups form after WHERE: 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

No hidden group work

The bucket table names the exact source rows in each group.

visible buckets\text{visible buckets}

Summary

GROUP BY works after the row filter has finished.

WHERE then GROUP BY\text{WHERE then GROUP BY}