Showing entries 1 to 3
Displaying posts with tag: Median (reset)
Greatest N per group: top 3 with GROUP_CONCAT()

In my opinion, one of the best things that happened to Planet MySQL lately, is Explain Extended, a blog by Alex Bolenok (also known as Quassnoi on Stackoverflow).

I never had the pleasure of meeting Alex in person, but his articles are always interesting and of high quality, and the SQL wizardry he pulls off is downright inspiring. I really feel humbled by the creativity of some of his solutions and his apparent experience with multiple RDBMS products.

Alex' most recent post is about aggregation, and finding a top 3 based on the aggregate:

In …

[Read more]
A fast, single pass method to calculate the median in MySQL

After stepping off of the GROUP_CONCAT() solution for calculating quantiles I figured it would be nice to find a better way to calculate the median too.
I previously wrote on how to calculate the median using GROUP_CONCAT(), but I think that this is a better way:

SELECT AVG(length) AS median -- take the average of left and right median
, MIN(length) AS left_median --
, MAX(length) AS right_median --
, @l AS left_median_position --
, @r AS right_median_position --
SELECT @n, length -- @n is just here to …
[Read more]
Calculating the Nth percentile in MySQL

Yesterday, I was on the freenode ##pentaho irc channel when Andres Chaves asked me how to calculate the Nth percentile in MySQL. He saw a solution somewhere using subqueries, but wasn't too happy about it.

A while ago I wrote about calulating the median in MySQL, and it turns out the Nth percentile can be calculated using a similar, single-pass approach, not relying on subqueries, UDFs, or user-defined variables.
The percentile....
So, what is a percentile exactly? Here's what the wikipedia says:

A percentile is the value of a variable below which a certain percent of observations fall. So the 20th percentile is the value (or score) below which 20 …

[Read more]
Showing entries 1 to 3