Discovered by: /uber-report 2026-05-10 (SqlBuildingBlocks QA)

Context

DBNull three-valued logic in WHERE/JOIN predicates was fixed in Wave 13 (issue #186). Unit-level coverage in QueryEngineTests.cs includes hand-built AST tests, but none of the integration tests in AnsiSqlScenarioTests.cs or MySqlScenarioTests.cs exercise nullable column predicates through a parsed SQL string. Hand-built AST tests bypass the grammar''s column-type inference path.

Proposed Work

Add to AnsiSqlScenarioTests.cs:

1. Scenario_NullableColumn_IsNull_FiltersCorrectly — WHERE col IS NULL returns only null rows
2. Scenario_NullableColumn_IsNotNull_FiltersCorrectly — WHERE col IS NOT NULL returns only non-null rows
3. Scenario_NullableColumn_EqualityWithNull_ExcludesNullRows — WHERE nullable_col = 5 excludes null rows (SQL three-valued logic)

Each test must use a parsed SQL string. Nullable column declared via typeof(int?) in AddTable to exercise the AllowDBNull unwrap path.

Acceptance Criteria

• All 3 tests parse SQL through the grammar and execute through QueryEngine
• Results match three-valued SQL logic
• All pass under dotnet test --configuration Release