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Median function in 10gR2

756666Feb 26 2010 — edited Feb 26 2010
I am seeing some unexpected behavior using the median function applied to a set of distinct values returned by a subquery. I didnt find an explanation by searching the web; hence the question here. These queries are on a 10gR2 database.

In both cases below, the select distinct should return 1,2,3,4,5; for a median of 3.

However the median is different when the 1,2 are repeated

This query returns 2 as median -

SELECT median(prob)
FROM (SELECT DISTINCT (prob) AS prob
FROM (SELECT 1 AS prob
FROM dual
UNION ALL
SELECT 2 AS prob
FROM dual
UNION ALL
SELECT 3 AS prob
FROM dual
UNION ALL
SELECT 4 AS prob
FROM dual
UNION ALL
SELECT 5 AS prob
FROM dual
UNION ALL
SELECT 1 AS prob
FROM dual
UNION ALL
SELECT 2 AS prob FROM dual) sub);

This query returns 3 as the median.

SELECT median(prob)
FROM (SELECT DISTINCT (prob) AS prob
FROM (SELECT 1 AS prob
FROM dual
UNION ALL
SELECT 2 AS prob
FROM dual
UNION ALL
SELECT 3 AS prob
FROM dual
UNION ALL
SELECT 4 AS prob
FROM dual
UNION ALL
SELECT 5 AS prob FROM dual) sub);

What am I missing about the median or the distinct function to cause this different behavior? My expectation was to get the same median - 3 - in both cases since the subquery that the median uses returns the same values in both.

Thanks in advance for any insight on this.

Comments

_AZ_

i think i should elaborate that I do expect to receive only one row ( from the select). Anything more (or less) should be deemed an error.

Answer

Well, with the code you have above you *will* get an error: TypeError: 'NoneType' object is not iterable. The reason for that is that fetchone() returns None if there are no rows left to fetch. That error isn't too helpful, though. You will need to do something along these lines:

row = cursor.fetchone()

if row is None:

   raise Exception("Hey, only one row was returned!")

tim, val = row

You will want to replace the Exception message with something a bit more meaningful, of course!

Marked as Answer by _AZ_ · Sep 27 2020
_AZ_

thank you @Anthony . Is there a better approach that i should.could use ( vs fetchone or overall ) ?

You're welcome. That approach works and is reasoanble. If you want to check for too many rows as well, you can do fetchall() which will return an array and check the length of the array instead. If you're worried about getting back too many rows with fetchall() you can also use fetchmany(2) which will tell you if there are 0, 1, or 2 rows available.

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Locked on Mar 26 2010
Added on Feb 26 2010
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