OP03: Number of people continuously observed throughout a year

Count number of people continuously observed throughout a specified year. 

Input:
 Parameter  Example  Mandatory  Notes
 START_DATE  01-jan-2011  Yes  Beginning of observation period inclusion
 END_DATE  31-dec-2011  Yes  End of observation period iclusion

Sample query run:
The following is a sample run of the query. The input parameters are highlighted in blue.
SELECT COUNT(DISTINCT person_ID) 
      AS NUM_persons 
FROM observation_period 
WHERE observation_period_start_date <= '01-jan-2011' 
      AND observation_period_end_date >= '31-dec-2011'
Output:
Output field list:
 Field  Description
num_persons Number of patients whose observation period within range of days

Sample output record:
 Field  Value
num_persons 5876507
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