OP04: Number of people who have gap in observation (two or more observations)

Count number of people who have two or more observations. 


Sample query run:
The following is a sample run of the query. 
SELECT count( person_id ) AS num_persons
  FROM -- more then one observatio period
     ( SELECT person_id
         FROM observation_period
        GROUP BY person_id
       HAVING COUNT( person_id ) > 1
Output field list:
 Field  Description
num_persons Number of patients who have two or more observations

Sample output record:
 Field  Value
num_persons 12695735