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### PE12: Distribution of year of birth

This query is used to to provide summary statistics for the age across all patient records: the mean, the standard deviation, the minimum, the 25th percentile, the median, the 75th percentile, the maximum. No input is required for this query.

Input:

<None>

Sample query run:
``````SELECT percentile_25
, median
, percentile_75
, MIN( year_of_birth )    AS minimum
, MAX( year_of_birth )    AS maximum
, CAST(AVG( year_of_birth ) AS INTEGER)   AS mean
, STDDEV( year_of_birth ) AS stddev

FROM
(SELECT MAX( CASE WHEN( percentile = 1 ) THEN year_of_birth END ) AS percentile_25
, MAX( CASE WHEN( percentile = 2 ) THEN year_of_birth END ) AS median
, MAX( CASE WHEN( percentile = 3 ) THEN year_of_birth END ) AS percentile_75
FROM -- year of birth / percentile
( SELECT year_of_birth, births
/* The first sum is the sum of all the values from the first year of birth
to the current year.  The second sum is the total of all the years of birth.
The result is a cumulative percent of the total for each year.  You want to
capture when the percentage goes from 24 to 25 as percentile_25, from 49 to 50
as the median and from 74 to 75 as the percentile_75.  Multiplying by 4 then
adding 1 just makes so that instead of looking at percentage, you get the whole
number 1 if the percentage is less than 25, 2 when the percentage is between 25
and 50, and so on.
*/
, FLOOR( CAST( SUM( births ) OVER( ORDER BY year_of_birth ROWS UNBOUNDED PRECEDING ) AS DECIMAL )
/ CAST( SUM( births ) OVER( ORDER BY year_of_birth ROWS BETWEEN UNBOUNDED PRECEDING
AND UNBOUNDED FOLLOWING )  AS DECIMAL )
* 4
) + 1 percentile
FROM -- Year with number of births
( SELECT year_of_birth, count(*) AS births
FROM person
GROUP BY year_of_birth
)
)where percentile <= 3
) percentile_table, person
GROUP BY percentile_25, median, percentile_75
``````
Output:
Output field list:
 Field Description percentile_25 25th percentile of year of birth median Median patient year of birth percentile_75 75th percentile of year of birth minimum Minimum year of birth maximum Maximum year of birth mean Mean patient year of birth stddev Standard deviation of year of birth

Sample output record:
 Field Value percentile_25 1954 median 1965 percentile_75 1979 minimum 1893 maximum 2010 mean 1968 stddev 17.277