This will work:
SELECT p_id, CONCAT(max(TreatmentDate) - min(TreatmentDate),' Days') as "Day Difference" FROM patient_info GROUP BY p_id;
Given this pattern / data:
CREATE TABLE patient_info ( p_id INT, TreatmentDate DATE ); INSERT INTO patient_info VALUES (15,'2008-05-01'), (15,'2008-05-03'), (15,'2008-05-05'), (15,'2008-05-07'), (16,'2008-05-01'), (16,'2008-05-03'), (16,'2008-05-05'), (16,'2008-05-09'), (17,'2008-05-03'), (17,'2008-05-05'), (17,'2008-05-07'); +
Please let me know if you need further assistance.
randy melder
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