คนไข้ที่มีปีเกิดน้อยกว่า 2121 และผลตรวจจากห้องปฏิบัติการเป็นน้ำไขสันหลัง (csf) มีกี่ราย?ไม่จำเป็นต้องใช้ทุกตาราง
what is the number of patients whose year of birth is less than 2121 and lab test fluid is cerebrospinal fluid (csf)?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.dob_year < "2121" AND lab.fluid = "Cerebrospinal Fluid (CSF)"