ขอทราบจำนวนผู้ป่วยที่มีสถานภาพสมรสเป็นโสด และชื่อตรวจในห้องปฏิบัติการเป็น po2 หรือไม่ ไม่จำเป็นต้องใช้ตารางทั้งหมด
give me the number of patients whose marital status is single and lab test name is po2?It is not neccessary to use all the tables.
 demographic:  subject_id [ text]  hadm_id [ text]  name [ text]  marital_status [ text]  age [ text]  dob [ text]  gender [ text]  language [ text]  religion [ text]  admission_type [ text]  days_stay [ text]  insurance [ text]  ethnicity [ text]  expire_flag [ text]  admission_location [ text]  discharge_location [ text]  diagnosis [ text]  dod [ text]  dob_year [ text]  dod_year [ text]  admittime [ text]  dischtime [ text]  admityear [ text] 
SELECT COUNT(DISTINCT demographic.subject_id) FROM demographic INNER JOIN lab ON demographic.hadm_id = lab.hadm_id WHERE demographic.marital_status = "SINGLE" AND lab.label = "pO2"