Single Column as N Rows [duplicate] - sql

Question

This question already has answers here:
Efficiently convert rows to columns in sql server
(5 answers)
Closed 7 years ago.
I have a large table with 3 columns as follows:
Invoice Product Color
1 Pant Red
1 Pant Black
1 Shirt Green
2 Pant White
2 Pant Black
2 Pant Blue
I'd like to group on Invoice & Product and then have all unique Color values appear on the related grouped record as follows:
Invoice Product Colour1 Colour2 Colour3
1 Pant Red Black
1 Shirt Green
2 Pant White Black Blue
Is this possible in SQL Server?

Answer 1

It is possible in SQL Server -- if you know that there are three color columns. If there are a variable number, then it is still possible, but it requires dynamic SQL.
I would approach this using conditional aggregation:
select invoice, product,
max(case when seqnum = 1 then colour end) as colour1,
max(case when seqnum = 2 then colour end) as colour2,
max(case when seqnum = 3 then colour end) as colour3
from (select t.*,
row_number() over (partition by invoice, product order by (select nULL)) as seqnum
from table t
) t
group by invoice, product;

Answer 2

To convert rows into columns, you need to use Pivot in Sql Server. If you know the number of columns in advance, you can use pivoting statically as the answer suggested by Gordin Linoff.
Sometimes, the number of colors may vary(in your example there are only 3 colors). In such case, you cannot hardcode the column names. For that first of all you need to get columns names dynamically into a variable.
DECLARE #cols NVARCHAR (MAX)
SELECT #cols = COALESCE (#cols + ',[' + COLUMNNAME + ']', '[' + COLUMNNAME + ']')
FROM
(
SELECT DISTINCT
'COLOR'+CAST(ROW_NUMBER() OVER(PARTITION BY INVOICE,PRODUCT ORDER BY (SELECT 0)) AS VARCHAR(10)) COLUMNNAME
FROM #TEMP
) PV
ORDER BY COLUMNNAME
Now the above variable have values of columns as Comma Separated Values which can be used with IN operator dynamically for the below query. Since your table doesn't have values like COLOR1, COLOR2 etc, I have provided logic to get column names for each INVOICE and its PRODUCT using PARTITION BY clause.
DECLARE #query NVARCHAR(MAX)
SET #query = '-- This outer query forms your pivoted result
SELECT * FROM
(
-- Source data for pivoting
SELECT DISTINCT INVOICE,PRODUCT,COLOR,
''COLOR''+CAST(ROW_NUMBER() OVER(PARTITION BY INVOICE,PRODUCT ORDER BY (SELECT 0)) AS VARCHAR(10)) COLUMNNAME
FROM #TEMP
) x
PIVOT
(
--Defines the values in each dynamic columns
MIN(COLOR)
-- Get the names from the #cols variable to show as column
FOR COLUMNNAME IN (' + #cols + ')
) p
ORDER BY INVOICE;'
EXEC SP_EXECUTESQL #query
Click here to view result