SQL view with dynamic count of columns

Question

I know it is impossible directly, but maybe with help of sql functions it is possible to create view with dynamic column count? What exactly I want to do is - Create view which\

Answer 1

I've made some assumptions about your data schema but it looks like you're trying to generate something like this

CREATE VIEW User_Properties AS
SELECT 
[username], [date]
,MAX(CASE WHEN [PropertyName] = 'property 1' THEN [PropertyValue]  ELSE NULL END) AS [property 1]
,MAX(CASE WHEN [PropertyName] = 'property 2' THEN [PropertyValue]  ELSE NULL END) AS [property 2]
,MAX(CASE WHEN [PropertyName] = 'property 3' THEN [PropertyValue]  ELSE NULL END) AS [property 3]
....
,MAX(CASE WHEN [PropertyName] = 'property n' THEN [PropertyValue]  ELSE NULL END) AS [property n]
GROUP BY [username], [date]

Which can be automated along the lines of

--Cursor to return the list of Properties
DECLARE PROPERTY_CURSOR CURSOR FOR SELECT PropertyName From UserPropertyTable

DECLARE @PropertyName nvarchar(255)
DECLARE @SQL nvarchar(max)

--Start Building the view definition
SET @SQL ='CREATE VIEW User_Properties AS
SELECT [username], [date]'

--Add a column for each Property
OPEN PROPERTY_CURSOR 
FETCH NEXT FROM PROPERTY_CURSOR INTO @PropertyName
WHILE @@FETCH_STATUS =0
BEGIN
SET @SQL = ',MAX(CASE WHEN [PropertyName] = '+QUOTENAME(@PropertyName,'''')+' THEN [PropertyValue]  ELSE NULL END) AS '+QUOTENAME(@PropertyName)
FETCH NEXT FROM PROPERTY_CURSOR INTO @PropertyName
END
DEALLOCATE PROPERTY_CURSOR 


--Finish off the Create_View SQL 
SET @SQL =@SQL +'
FROM UserPropertyTable
GROUP BY [username], [date]'

--Execute the Create_View SQL 
EXEC (@SQL)

Answer 2

This is not possible in standard SQL, nor in any version of SQL that I am familiar with (SQL Server, Oracle, MySql, Access SQL). Tables, Expressions and Views in SQL have a fixed column-set, by design. I.E., it's intentionally restricted this way. AFAIK, in most versions of SQL, Stored Procedures are the only objects that can return a variable column-set.