Among the patients with hypertension, what is the average of their diastolic blood pressure?
hypertension refers to conditions where DESCRIPTION = 'Hypertension'; average diastolic blood pressure refers to AVG(VALUE) where DESCRIPTION = 'Diastolic Blood Pressure' from observations;
SELECT AVG(T1.VALUE) FROM observations AS T1 INNER JOIN patients AS T2 ON T1.PATIENT = T2.patient INNER JOIN conditions AS T3 ON T2.patient = T3.PATIENT WHERE T3.DESCRIPTION = 'Hypertension' AND T1.DESCRIPTION = 'Diastolic Blood Pressure'