How do i get min, median and max from my query in postgresql

Question

I have written a query in which one column is a month. From that I have to get min month, max month, and median month. Below is my query.

select ext.employee         


        

Answer 1

You want the aggregate functions named min and max. See the PostgreSQL documentation and tutorial:

• http://www.postgresql.org/docs/current/static/tutorial-agg.html
• http://www.postgresql.org/docs/current/static/functions-aggregate.html

There's no built-in median in PostgreSQL, however one has been implemented and contributed to the wiki:

http://wiki.postgresql.org/wiki/Aggregate_Median

It's used the same way as min and max once you've loaded it. Being written in PL/PgSQL it'll be a fair bit slower, but there's even a C version there that you could adapt if speed was vital.

UPDATE After comment:

It sounds like you want to show the statistical aggregates alongside the individual results. You can't do this with a plain aggregate function because you can't reference columns not in the GROUP BY in the result list.

You will need to fetch the stats from subqueries, or use your aggregates as window functions.

Given dummy data:

CREATE TABLE dummystats ( depname text, empno integer, salary integer );
INSERT INTO dummystats(depname,empno,salary) VALUES
('develop',11,5200),
('develop',7,4200),
('personell',2,5555),
('mgmt',1,9999999);

... and after adding the median aggregate from the PG wiki:

You can do this with an ordinary aggregate:

regress=# SELECT min(salary), max(salary), median(salary) FROM dummystats;
 min  |   max   |         median          
------+---------+----------------------
 4200 | 9999999 | 5377.5000000000000000
(1 row)

but not this:

regress=# SELECT depname, empno, min(salary), max(salary), median(salary)
regress-# FROM dummystats;
ERROR:  column "dummystats.depname" must appear in the GROUP BY clause or be used in an aggregate function

because it doesn't make sense in the aggregation model to show the averages alongside individual values. You can show groups:

regress=# SELECT depname, min(salary), max(salary), median(salary) 
regress-# FROM dummystats GROUP BY depname;
  depname  |   min   |   max   |          median          
-----------+---------+---------+-----------------------
 personell |    5555 |    5555 | 5555.0000000000000000
 develop   |    4200 |    5200 | 4700.0000000000000000
 mgmt      | 9999999 | 9999999 |  9999999.000000000000
(3 rows)

... but it sounds like you want the individual values. For that, you must use a window, a feature new in PostgreSQL 8.4.

regress=# SELECT depname, empno, 
                 min(salary) OVER (), 
                 max(salary) OVER (), 
                 median(salary) OVER () 
          FROM dummystats;

  depname  | empno | min  |   max   |        median         
-----------+-------+------+---------+-----------------------
 develop   |    11 | 4200 | 9999999 | 5377.5000000000000000
 develop   |     7 | 4200 | 9999999 | 5377.5000000000000000
 personell |     2 | 4200 | 9999999 | 5377.5000000000000000
 mgmt      |     1 | 4200 | 9999999 | 5377.5000000000000000
(4 rows)

See also:

• http://www.postgresql.org/docs/current/static/tutorial-window.html
• http://www.postgresql.org/docs/current/static/functions-window.html