Context

Surfaced by Wave 11 (#168 recursive CTE; commit 0c69604).

The QueryEngine's expression-builder cannot compare a DataRow column whose runtime value is DBNull to an int operand inside a JOIN ON predicate. Specifically: JOIN parent ON child.ParentId = parent.Id where ParentId is nullable and the parent column type is int — when ParentId is NULL, the comparison throws or produces incorrect results instead of evaluating to NULL (and being excluded by the ON predicate per SQL three-valued logic).

This forced Wave 11's recursive CTE tests to use sentinel 0 for "no parent" instead of NULL. Real-world hierarchy data (org charts, tree structures) typically uses NULL for the root, so this is a practical gap.

Repro

Engineering hierarchy table:

Id | ParentId
 1 | NULL
 2 | 1
 3 | 2

Query:

WITH RECURSIVE chain AS (
  SELECT Id, ParentId FROM Engineers WHERE ParentId IS NULL
  UNION ALL
  SELECT e.Id, e.ParentId FROM Engineers e JOIN chain c ON e.ParentId = c.Id
)
SELECT * FROM chain

Expected: 3 rows (the entire chain).
Actual: NULL/Int promotion error in the JOIN ON predicate's expression compilation.

Diagnosis pointer

src/Core/QueryProcessing/QueryEngine.cs LINQ expression builder for JOIN ON predicates. The runtime value extracted from DataRow[col] is object, which becomes DBNull for NULL rows. The expression builder needs to compare-or-fail-fast on DBNull rather than attempting an int conversion.

SQL three-valued logic semantics: any comparison involving NULL evaluates to NULL (UNKNOWN), which is not TRUE, so the ON predicate excludes the row. The expression builder must implement this short-circuit.

Acceptance Criteria

• JOIN ON predicate with a nullable column compared to a non-null operand correctly evaluates to NULL/UNKNOWN, excluding the row from the join result
• Recursive CTE hierarchy traversal with NULL "no parent" sentinel works end-to-end
• Regression tests covering: nullable column = nullable column, nullable column = non-null literal, JOIN ON NULL = NULL (which should NOT match per SQL semantics)
• No regression on existing JOIN tests

Priority

P1 (NULL-handling correctness gap; common pattern in real-world data; produces silently-wrong results today).

Discovered by

Wave 11 of /uber-report 2026-05-09 (sqlbuildingblocks-developer).