COALESCE

Column Fallback

A later column can fill a NULL first choice.

highlighted = computed this step

Column fallback

If the first column is NULL, COALESCE can try another column.

try the next column\text{try the next column}

Read fallback rows

A later argument supplies the result for 3 rows.

fallback rows=3\text{fallback rows}=3

COALESCE examples are tiny finite row transforms with SQL-style NULL choice behavior; SQL dialect completeness, type coercion, optimizer behavior, indexing, and product claims are out of scope.

Column fallback: base rowsnameemailbackupnicknameAdaada@exampleada@backupNULLBeaNULLbea@backupBCalNULLNULLCalDeeNULLNULLNULL COALESCE argument choicesourcearg valueschosenIndexresultresultSource0[ada@example, ada@backup, missing]0ada@exampleemail1[NULL, bea@backup, missing]1bea@backupbackup2[NULL, NULL, missing]2missingliteral3[NULL, NULL, missing]2missingliteral COALESCE output rowsnameemailbackupnicknamecontactAdaada@exampleada@backupNULLada@exampleBeaNULLbea@backupBbea@backupCalNULLNULLCalmissingDeeNULLNULLNULLmissing COALESCE factsfactvalueinputRowCount4argCount3outputRowCount4firstArgRows1fallbackRows3literalDefaultRows2allNullRows0rowOrdersource_orderwhereContrastvalue_choice_not_truth_filter

Backup can fill a missing email

Bea has no email value, so the backup column becomes the contact value.

email NULL then backup\text{email NULL then backup}

Summary

A fallback column is still just another ordered argument.

ordered fallback\text{ordered fallback}