How many patients have a normal anti-DNA level, yet their data are not recorded.
normal anti-DNA level refers to DNA < 8; data are not recorded refers to Description IS NULL; Should compute the number of unique ones
SELECT COUNT(DISTINCT T1.ID) FROM Patient AS T1 INNER JOIN Laboratory AS T2 ON T1.ID = T2.ID WHERE T2.DNA < 8 AND T1.Description IS NULL