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Displaying posts with tag: User defined variable (reset)
MySQL User Defined Variables

When I have discovered MySQL User-defined variables at the first time, I didn’t pay much attention to them and didn’t realize their strength. At first, one can think that they are just one of the many unnoticed MySQL features. Well, I was wrong. Here, I will try to summarize their strength, though I think there is […]

MySQL: Another Ranking trick

I just read SQL: Ranking without self join, in which Shlomi Noach shares a nice MySQL-specific trick based on user-defined variables to compute rankings.

Shlomi's trick reminds me somewhat of the trick I came across little over a year ago to caclulate percentiles. At that time, several people pointed out to me too that using user-defined variables in this way can be unreliable.The problem with user-defined variablesSo what is the problem exaclty? Well, whenever a query assigns to a variable, and that same variable is read in another part of the query, you're on thin ice. That's because the …

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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 …
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Calculating Percentiles with MySQL, Round 2

My previous post on calculating percentiles with MySQL generated some comments and good discussion. In particular, I got some very interesting comments from Vladimir.

Basically, Vladimir was doubtful whether the GROUP_CONCAT() solution would be optimal in comparison to a JOIN. His proposal is to solve it like this:

SELECT SUM(g1.r) sr
, g2.length l
, SUM(g1.r)/(SELECT COUNT(*) FROM film) p
FROM (SELECT COUNT(*) r, length FROM film GROUP BY length) g1
JOIN (SELECT COUNT(*) r, length FROM film GROUP BY length) g2
ON g1.length < g2.length
GROUP BY g2.length
HAVING p > 0.9

First, this query sets up two identical subqueries in …

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Showing entries 1 to 4