How to compute column by specific value? - mysql

Question

How to query result like these picture.
First column select is this month plus next column by condition field (SelectColumn)
yellow background is the select column for sum
my example code.
declare #myDate date = getdate(),#qry varchar(max)
set #qry = 'select case v.SelectColumn
when 0 then (SELECT '+DATENAME(month,#myDate)+')
when 2 then (SELECT '+DATENAME(month,#myDate)+'+'+DATENAME(MONTH,DATEADD(MONTH,1,#myDate))+')
when 1 then (SELECT '+DATENAME(month,#myDate)+'+'+DATENAME(MONTH,DATEADD(MONTH,1,#myDate))+'+'+DATENAME(MONTH,DATEADD(MONTH,2,#myDate))+')
end
as SumColumn
from vwQC12Month v'
exec (#qry)

Answer 1

This problem requires the monthly values to be accessible as rows such that selected aggregation can be applied. Typically, you can use UNPIVOT or VALUES to rotate columns into rows. Below is a working example using UNPIVOT.
In the query below, you'd have to change the short form month names to match your column names for it to work. Here is a working demo of this: http://sqlfiddle.com/#!18/094e4/2
Also you might want to consider how you want to deal with year end wrapping and adjust the aggregation accordingly.
-- setup sample data
create table Test (
id int, Jan int, Feb int, Mar int, Apr int, May int,
Jun int, Jul int, Aug int, Sep int, Oct int, Nov int, Dec int,
SelectColumn int)
insert Test values
(1, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 1),
(2, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 2),
(3, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 3),
(4, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 4)
-- query
DECLARE #currentMonthNumber int = MONTH(getdate())
-- CTE that rotates the data into rows
; WITH RowsByMonth(id, SelectColumn, monthNumber, val) AS
(
SELECT id, SelectColumn, CONVERT(int, month) AS monthNumber, val
FROM
(SELECT id, SelectColumn,
Jan AS [1], Feb AS [2], Mar AS [3], Apr AS [4],
May AS [5], Jun AS [6], Jul AS [7], Aug AS [8],
Sep AS [9], Oct AS [10], Nov AS [11], Dec AS [12]
FROM Test) AS Source
UNPIVOT
(val FOR month IN
([1], [2], [3], [4],
[5], [6], [7], [8],
[9], [10], [11], [12])
) AS asRows
) -- aggregation below
SELECT id, SUM(val) AS SumAcrossSelectedMonths
FROM RowsByMonth
WHERE
monthNumber >= #currentMonthNumber
AND monthNumber - #currentMonthNumber < SelectColumn
GROUP BY id
-- Results
| id | SumAcrossSelectedMonths |
|----|-------------------------|
| 1 | 40 |
| 2 | 90 |
| 3 | 150 |
| 4 | 220 |