รับจำนวนผู้ป่วยอายุต่ำกว่า 50 ปี ที่มีประเภทการทดสอบในห้องปฏิบัติการเคมี ไม่จำเป็นต้องใช้ทุกตาราง
get the number of patients less than 50 years of age who have chemistry lab test category.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.age < "50" AND lab."CATEGORY" = "Chemistry"