The Inequality Filter That Drops Your NULL Rows
A WHERE inequality drops rows where the column is NULL, because the comparison is UNKNOWN. When to add OR col IS NULL, and when not to.
NOT IN With a NULL Returns Nothing
One NULL in a NOT IN subquery makes the whole predicate UNKNOWN and returns zero rows. Why it happens and how NOT EXISTS fixes it.
The MERGE That Would Not Update: A NULL in Your Change Detection
A MERGE upsert skips a NULL-to-value change and updates nothing, with no error. The three-valued-logic cause and two NULL-safe fixes.