Try:
SELECT
(SELECT (COUNT(1)*100) from myTable where ISNULL(myColumn,'')='')
/
(SELECT COUNT(1) FROM myTable)
"Thanks sir for your reply, but i need to count all the columns(columns having value) of a single row and show it in %"
Your original question asked for:
"return as a percentage the number of fields in a row that are null"
If you have 6 rows, with null in two of them, the query above will return "33" - ie, 33% or 1/3
To invert it, and return the percentage that are not null, put a "NOT" in front of the ISNULL:
SELECT
(SELECT (COUNT(1)*100) from myTable where NOT ISNULL(myColumn,'')='')
/
(SELECT COUNT(1) FROM myTable)