Select not null columns as a comma separated string from dynamic sql - sql

Question

I am trying to do my best to avoid using cursors. There is a comma separated list of columns in nvarchar variable that looks like this:
#columnList = 'col1,col2,col5'
There is a table with lots of varchar columns:
myTable: [col1],[col2],[col3],[col4],[col5],[col6],[col7]
This is how I select data using a dynamic sql:
exec ('select ' + #columnList + ' from myTable')
This query returns the following results:
[col1], [col2] , [col5]
null , "txt1" , null
"txt2", null , null
null , "txt3" , "txt4"
This is what I need to get:
#resultList = "txt1,txt2,txt3,txt4"
How do I select a comma separated string containing not-null values only? I know how to convert a table to comma separated string, so getting something like:
[column]
"txt1"
"txt2"
"txt3"
"txt4"
Is also fine. Any suggestions? Feel free to suggest a basic approach, I don't expect you to write the actual code for me.

Answer 1

You can use a solution like the following, using just a REPLACE to create the SQL query:
DECLARE #columnList VARCHAR(100)
SET #columnList = 'col1,col2,col5'
SET #columnList = REPLACE(#columnList, ',', ' AS colName FROM myTable UNION ALL SELECT ');
EXEC('SELECT * FROM (SELECT ' + #columnList + ' AS colName FROM myTable)t WHERE NOT t.colName IS NULL');
You can also use a solution using UNPIVOT:
DECLARE #columnList VARCHAR(100);
SET #columnList = 'col1,col2,col5';
EXEC('SELECT colName FROM (SELECT ' + #columnList + ' FROM myTable) t1 UNPIVOT (colName FOR columnNames IN (' + #columnList + ')) AS t2');
demo on dbfiddle.uk

Answer 2

Since you mention you already know how to aggregate into comma-seperated, here's how to unpivot your table with cross apply:
select unpivoted.*
from myTable
cross apply
( values
('col1',col2)
,('col2',col2)
,('col3',col3) -- etc
)unpivoted(colname,colval)

Answer 3

This would work for SQL Server 2017, on earlier version STRING_AGG usage should be replaced with XML-based solution.
You could first concatenate your results into a single line and then use STRING_AGG to aggregate them into a single string:
;with t as
(
SELECT * FROM (VALUES
(null , 'txt1' , null),
('txt2', null , null),
(null , 'txt3' , 'txt4')) x(col1, col2, col5)
)
SELECT STRING_AGG(STUFF(CONCAT(',' + col1, ',' + col2, ',' + col5), 1, 1, ''), ',')
FROM t
CTE is just for showcasing, you can simply do it in your dynamic query:
DECLARE #columnList NVARCHAR(MAX) = 'col1,col2,col5'
DECLARE #query NVARCHAR(MAX) = ''
SELECT #query = 'SELECT STRING_AGG(STUFF(CONCAT('','' + ' + REPLACE(#columnList, ',', ', '','' + ') + '), 1, 1, ''''), '','') from mytable'
EXEC sp_executesql #query
Working example on dbfiddle