instruction: what's patient 031-23724's sao2 difference second measured on the first intensive care unit visit compared to the first value measured on the first intensive care unit visit?
response: SELECT (SELECT vitalperiodic.sao2 FROM vitalperiodic WHERE vitalperiodic.patientunitstayid IN (SELECT patient.patientunitstayid FROM patient WHERE patient.patienthealthsystemstayid IN (SELECT patient.patienthealthsystemstayid FROM patient WHERE patient.uniquepid = '031-23724') AND NOT patient.unitdischargetime IS NULL ORDER BY patient.unitadmittime LIMIT 1) AND NOT vitalperiodic.sao2 IS NULL ORDER BY vitalperiodic.observationtime LIMIT 1 OFFSET 1) - (SELECT vitalperiodic.sao2 FROM vitalperiodic WHERE vitalperiodic.patientunitstayid IN (SELECT patient.patientunitstayid FROM patient WHERE patient.patienthealthsystemstayid IN (SELECT patient.patienthealthsystemstayid FROM patient WHERE patient.uniquepid = '031-23724') AND NOT patient.unitdischargetime IS NULL ORDER BY patient.unitadmittime LIMIT 1) AND NOT vitalperiodic.sao2 IS NULL ORDER BY vitalperiodic.observationtime LIMIT 1)