NULL Tests

Select Missing Values

IS NULL selects missing values.

highlighted = computed this step

Rows with missing values

The same IS NULL shape can test a different column.

test another column\text{test another column}

Read missing score rows

The score IS NULL test outputs 1 row.

missing score rows=1\text{missing score rows}=1

NULL-test examples are tiny finite-table transforms for IS NULL and IS NOT NULL; SQL dialect completeness, optimizer behavior, indexing, and product claims are out of scope.

Select rows with missing values: base rowsidnamecityscore1AnnAustin102BoNULL203CyBostonNULL4DiNULL305EliDenver40 NULL test result per rowsourcetestedValueisNulltruthoutputRole010noFALSEdropped120noFALSEdropped2NULLyesTRUEkept330noFALSEdropped440noFALSEdropped WHERE-style TRUE-only output rowsidnamecityscore3CyBostonNULL NULL-test factsfactvalueinputRowCount5nullCount1presentCount4trueCount1falseCount4unknownCount0outputRowCount1droppedRowCount4negatednowhereKeepsTRUE_only_source_order

Missing score is visible

The output row is the row whose score cell is NULL.

missing score kept\text{missing score kept}

Summary

IS NULL selects missing values without guessing a replacement value.

no default guessed\text{no default guessed}