Provide the social security number of the patient with the highest systolic blood pressure.
social security number refers to ssn; highest systolic blood pressure refers to MAX(observations.VALUE WHERE observations.DESCRIPTION = 'Systolic Blood Pressure');
SELECT T2.ssn FROM observations AS T1 INNER JOIN patients AS T2 ON T1.PATIENT = T2.patient WHERE T1.DESCRIPTION = 'Systolic Blood Pressure' ORDER BY T1.VALUE DESC LIMIT 1