instruction: provide the number of patients whose diagnosis icd9 code is 4142 and lab test fluid is joint fluid.
response: SELECT COUNT(DISTINCT demographic.subject_id) FROM demographic INNER JOIN diagnoses ON demographic.hadm_id = diagnoses.hadm_id INNER JOIN lab ON demographic.hadm_id = lab.hadm_id WHERE diagnoses.icd9_code = "4142" AND lab.fluid = "Joint Fluid"