Among the patients who have a normal platelet level, how many of them have other symptoms observed?
normal platelet level refers to PLT > 100 and PLT < 400; have other symptoms refers to Diagnosis is not null;
SELECT COUNT(T1.ID) FROM Patient AS T1 INNER JOIN Laboratory AS T2 ON T1.ID = T2.ID WHERE T2.PLT BETWEEN 100 AND 400 AND T1.Diagnosis IS NOT NULL