How to divide two values from the same column in SQL - sql

Question

As the title states I would like to divide two values with each other that are in the same column.
E.g.
A B C D
Shirts 2011 85 0
Shirts 2012 92 percent change from 2011 to 2012
Shirts 2013 100 percent change from 2012 to 2013
Pants 2011 31 0
Pants 2012 42 percent change from 2011 to 2012
Pants 2013 55 percent change from 2012 to 2013
Jacket 2011 10 0
Jacket 2012 16 percent change from 2011 to 2012
Jacket 2013 18 percent change from 2012 to 2013
In this example column D would be a derived from column C, where the value of 2012 is subtracted from 2011, and then times by a 100 to get the percent.
I don't know how to set the query up I tried doing a bunch of sub-queries but didn't know how to link them together. Any help would be greatly appreciated.

Answer 1

Here's an option to get what you're after
SELECT t1.a, t1.b, t1.c,
CASE WHEN t2.c IS NULL THEN 0 ELSE t1.c - t2.c END AS d,
CASE WHEN t2.c IS NULL THEN 0 ELSE (1.0 * t1.c - t2.c) / t1.c * 100.0 END AS pct
FROM t t1
LEFT OUTER JOIN t t2 ON t1.a = t2.a
AND t1.b = t2.b + 1
SQL Fiddle Example

Answer 2

If I'm understanding your requirements correctly, you could use a recursive cte to accomplish your results. This uses ROW_NUMBER() and partitions by (groups by) column A.
This method would best work if you can't guarantee sequential years in column B. If you always have sequential years, then bobs provides the best alternative.
with cte as (
select A, B, C,
ROW_NUMBER() OVER (PARTITION BY A ORDER BY B) rn
from yourtable
),
recursive_cte as (
select A, B, C, 0 prc
from cte
where rn = 1
union all
select y.A, y.B, y.C, y.C-c.C prc
from cte y
join cte c on y.a=c.a and y.rn=c.rn+1
)
select *
from recursive_cte
order by a, b
SQL Fiddle Demo
This will return the difference between each year and the previous grouped by column A. It uses a 2nd CTE just for simplicity. If you want the actual percent changed, update the formula above.
--EDIT
Since it sounds like you're looking for percentage growth, try replacing y.C-c.C prc with this formula instead:
cast(y.C as decimal(10,2))/cast(c.C as decimal(10,2))
Use CAST if your data types are integers.

Answer 3

You can accomplish this with a self-join:
SELECT curr.ClothingType, curr.SalesYear, curr.NumberSold
, COALESCE((CAST((curr.NumberSold - prev.NumberSold) AS NUMERIC(5,2)) / prev.NumberSold) * 100, 0) AS PercentChange
FROM #Clothes AS curr
LEFT JOIN #Clothes AS prev ON curr.ClothingType = prev.ClothingType
AND curr.SalesYear = prev.SalesYear + 1
SQL Fiddle Example

Answer 4

I agree with Shivan, but only because I don't know how to do it without PHP. With PHP it's just a matter of pulling the two totals, doing the math and sending a query back with the answers.
ALSO BE SURE TO READ THIS BECAUSE IT'S THE MORE IMPORTANT PART
Your math's wrong. (Shirts2011 - Shirts2012) * 100 doesn't give you the percent change. It just gives you 100 times the difference of the two (actually, the negative difference based on your wording). What you want, math-wise, is:
((Item2012-Item2011)/Item2011)*100
I know the outside parens aren't necessary but they make it easier to read. The percent change is the same as 100 times the ratio of the difference of the two to the initial.
Edited to add PHP code.
The PHP looks gross, but it gets the job done.
$conn = new PDO(HOST:DATABASE, USERNAME, PASSWORD);
$query = "SELECT A, B, C FROM yourTable";
$st = $conn->prepare($query);
$st->execute();
$list = array();
while($row = $st->fetch(PDO::FETCH_ASSOC))
$list[] = $row;
$fill = array();
for($i=1; $i<count($list); $i++)
if($list[$i]['A'] == $list[$i-1][A])
$fill[] = $array('A' => $list[$i]['A'],
'B' => $list[$i]['B'],
'D' => ($list[$i]['C']-$list[$i-1]['C'])/$list[$i-1]['C']*100);
$update = new PDO(HOST:DATABASE, USERNAME, PASSWORD);
for($i=0; $i<count($fill); $i++){
$query = "UPDATE yourTable SET D = " . $fill[$i]['D'] . " WHERE A = " . $fill[$i]['A'] . " AND B = " . $fill[$i]['B'];
$st = $update->prepare($query);
$st->execute();
}
Just initialize your table where the 2011 D columns = 0.
UPDATE yourTable SET D = 0 WHERE B = 2011;
And before the flames start, I know a foreach would have worked; I just think a standard for looks better (and allows you to skip the first 2011 value).

Answer 5

Try this:
SELECT t1.type A, t1.year B, t1.value C,
IF(
EXISTS(
SELECT 1 FROM table t2 WHERE t1.type = t2.type AND t1.year = (t2.year - 1)
),
((SELECT t3.year FROM table t3 WHERE t1.type = t3.type AND t1.year = (t3.year - 1)) - t1.year)/t1.year,
0
) D
FROM table t1
Not super efficient, but it can be done.

Answer 6

You could try this too:
UPDATE mytable t1
INNER JOIN mytable t2
ON t1.A = t2.A AND t2.B = t1.B - 1
SET t1.D = ((t1.C - t2.C) / t2.C) * 100;
see fiddle.
Assuming you already have 0 as a default for D. If not, just set it to 0 for all rows before running this query.

Answer 7

Try this:
select Y2013.*, Y2012.C, Y2013.C, Y2013.C/Y2011.C
from (
select * from data where B = year(getdate())
) Y2013
left join (
select * from data where B = year(getdate()) - 1
) Y2012 on Y2012.A = Y2013.A
left join (
select * from data where B = year(getdate()) - 2
) Y2011 on Y2011.A = Y2012.A
and Y2011.A = Y2013.A