NULL Isn't Equal to NULL — Python Bug Hunt

Briefing

Inspired by the SQL lesson everyone learns in production: WHERE col = NULL matches nothing, ever — three-valued logic demands IS NULL. The report for "orders without a shipping date" has been returning zero rows for a month.

where_builder.py generates the clause; the locked engine follows real SQL semantics.

Bug report

BUG-3VL · Priority: High · Reported by: analytics

build_where(field, value):

• value is None -> {"clause": "<field> IS NULL", "params": []}
• otherwise -> {"clause": "<field> = ?", "params": [value]}

Observed: None becomes "= ?" with a NULL param — the engine (correctly) matches nothing, and ops thinks every order shipped.

Logs

[report] unshipped orders: 0 (warehouse says: definitely not zero)

The code as shipped

src/sql/where_builder.py (editable)

# Builds a WHERE clause for a single field.

def build_where(field, value):
    return {"clause": field + " = ?", "params": [value]}

Read-only context: src/sql/sqlengine.py.