From: Mark Haney
Date: September 7 2012 1:11pm
Subject: Re: Create a VIEW with nested SQL
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On 09/06/2012 10:23 AM, hsv@stripped wrote:

> SELECT lights.*, machine.mach_name
> FROM lights JOIN machine USING (mach_id)
> /* ORDER BY date DESC */
> GROUP BY mach_id
>
> ? With USING the fields "mach_id" from "lights" and "machine" become one unambiguous field "mach_id".
>
> Does "mach_id" really occur more times in "lights" or "machine"? If only once in both tables, GROUP BY at most orders it. It is not clear that ORDER BY does anything with GROUP BY outside it, unless "mach_id" is unique in both tables--I have found that GROUP BY not always orders the output, when everything is unique.
>
> And yes, MySQL balks at saving a view with a query for a table. One has to make them separate views.
>
>

The problem I encountered is that I can't find a way to just pull the 
most recent records for each machine without the GROUP BY statement. 
That's all I need.

Okay, so here's what I tried.  I created a view vLights from:


> SELECT MAX(lights.date),machine.mach_name FROM lights JOIN machine USING (mach_id) GROUP BY mach_name;

Now, as I use the query by itself I get this:

> mysql> SELECT MAX(lights.date),machine.mach_name FROM lights JOIN machine USING (mach_id) GROUP BY mach_name;
> +---------------------+-----------+
> | MAX(lights.date)    | mach_name |
> +---------------------+-----------+
> | 2012-09-07 09:03:10 | #10       |
> | 2012-09-07 09:03:07 | #12       |
> +---------------------+-----------+
> 2 rows in set (3.62 sec)

This is better than the 20s+ I was getting before, but still not 
acceptable for only 2 machines when I'll have 40+ at production time.

All I need is the most recent record for EACH machine ID, THEN to pull 
the machine name from the table that has the name in it.

Somehow I'm missing something incredibly obvious here.

-- 

Mark Haney
Software Developer/Consultant
AB Emblem
markh@stripped
Linux marius.homelinux.org 3.5.1-1.fc17.x86_64 GNU/Linux