Gary,
You have a right to be confused - this problem is short but it is about as
complex as they get. Still I have a solution for you, also short and you'll
(hopefully) find it elegant. I found it interesting...
The thing is we will create a Cartesian product on purpose - this is
something they tell people learning SQL to avoid (and usually they are not
helpful) but here it is the only to get a daily average since you don't have
daily records of your pairs just a start and stop date.
I was looking at the problem from the child point of view in my first post
(easier since children only have a single birthday). I'll use your problem
now.
We will keep the DateTable and use your 'breeding' table but I will change
the 'paired' and 'split' columns to int (you don't need to do this but it
will make thing easier - some DBMS's will always put a time stamp in a Date
field and this does screw thing up - it makes it harder to join on them).
So we have:
breeding
-----------
rec SMALLINT PRIMARY KEY
sire SMALLINT
dam SMALLINT
paired INT
split INT
num_offspring INT
And
DateTable
------------
dayID INT PRIMARY KEY
myDate DATE
month INT
year INT
quarter INT
/* and as discussed any other date related columns */
> I need to count how many pairs were breeding
> on a given day, and then average over the month.
OK - fasten you seat belt :-0
SELECT count(*), myDate
FROM breeding, DateTable
WHERE paired >= dayID
AND split <= dayID
AND month = 2
AND year = 2001
GROUP BY myDay
ORDER BY myDay
/* you could have just as easily used a start and stop date */
/* myDay >= "2/1/2001" and myDate < "3/1/2001" */
This will give you output something like this (the total per day):
5 Feb 1, 2001
5 Feb 2, 2001
5 Feb 3, 2001
5 Feb 4, 2001
5 Feb 5, 2001
6 Feb 6, 2001
6 Feb 7, 2001
6 Feb 8, 2001
6 Feb 9, 2001
6 Feb 10, 2001
6 Feb 11, 2001
... And so on ...
To get the average per month I'd use a temporary table (a table you create
just for this report or that is always there but empty except during this
report)
/* start with an empty table */
CREATE TABLE TotalByDay
( total INT,
dayID INT)
/* --- or --- */
TRUNCATE TABLE TotalByDay
/* now fill it... */
/* notice it the same SQL but I've gone back to */
/* dayID (the INT) not the full date and I've done */
/* the whole year so we can average per month */
INSERT INTO TotalByDay
SELECT count(*), dayID
FROM breeding, DateTable
WHERE paired >= dayID
AND split <= dayID
AND year = 2001
GROUP BY dayID
ORDER BY dayID
/* OK almost there ... Now to average by month */
SELECT ave(total), month, year
FROM TotalByDay, DateTable
WHERE TotalByDay.dayID = DateTable.dayID
GROUP BY month
ORDER BY month
Now you've got output like
5 1 2001
6 2 2001
6 3 2001
7 4 2001
12 5 2001
11 6 2001
11 7 2001
7 8 2001
7 9 2001
9 10 2001
9 11 2001
9 12 2001
>From here you convert the month INT back into a name.
Feel free to writ back with any questions (it's headdy stuff) also I don't
have a mySQL db handy to check the syntax against - hopefully I've
remembered everything correctly. The concept is sound, we do this all the
time.
The "got ya's" are the DateTable must be full (don't fill it to the end of
this year and forget about it - next year your reports will be wrong)
Frank
>
> Ouch, I'm getting a bit lost. I need to count how many pairs were breeding
> on a given day, and then average over the month. Playing with mysql I get
> for say the 1st day of the year:
>
> select count(*) from breeding
> where TO_DAYS(paired) <= TO_DAYS('2002-1-1')
> AND TO_DAYS('2002-1-1') <= TO_DAYS(split);
>
> That will give me the number of pairs breeding that day. Then I would have
> to loop through the year and keep track of number of pairs, days of months
> etc to get the monthly averages.
>
> Can you suggest a DateTable format that could help me? The start and end
> dates for the query would be input via a form. The granularity would be
> one month, but the details need to be averaged on a daily basis.
>
> Regards,
> Garry.
>
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