Dynamically insert column names into function - sql

Question

I try to insert column names into my query to obtain the max-length of the underlying data-input, like:
DECLARE #cols AS varchar(MAX)
,#query AS nvarchar(MAX)
SELECT #cols = (
select STRING_AGG(COLUMN_NAME, ',')
from INFORMATION_SCHEMA.COLUMNS
where TABLE_NAME='mytbl'
and TABLE_SCHEMA = 'dbo'
and DATA_TYPE = 'varchar'
)
set #query = 'select top(1) max(datalength(' + #cols + ')) FROM dbo.mytbl'
exec sp_executesql #query;
.. which prompts the following error:
The datalength function requires 1 argument(s).
I don´t know how to cast the string so that the #cols are recognized by the encasing max(datalength()) function(s).
Any ideas how to circumvent this error? I am out of ideas.
UPDATE:
The intended output should look something like that:
select max(datalength(col1))
,max(datalength(col2))
,max(datalength(col3))
,max(datalength(col4))
...
from dbo.mytable
which works and gives me the max-char of any input per column, like:

Answer 1

you need to apply the MAX() and DATALENGTH() on the column before concatenation
Change the #cols to
SELECT #cols = (
select STRING_AGG('MAX(DATALENGTH(' + COLUMN_NAME + '))', ',')
from INFORMATION_SCHEMA.COLUMNS
where TABLE_NAME='mytbl'
and TABLE_SCHEMA = 'dbo'
and DATA_TYPE = 'varchar'
)
and the final #query is just a simple select
set #query = 'select ' + #cols + ' FROM dbo.mytbl'
You should print out the query for inspection, before execute it
print #query