Isn't these two code snippets for SQL SERVER return same value

Question

In my sql server I got a table named "Product" where a column named "P_name" exist. This column is completely null meaning it has null value in all of its rows eg:

P_name
NULL
NULL
NULL

I wrote two queries. My first query is:

declare @value int
select value = count(P_name) from Product

It returned the value of 0 (which is correct because the P_name column is completely null)

I wrote the second query as follows:

declare @colName nvarchar(max) = 'P_name';
declare @value int
select value = count(@colName) from Product

This returned a value with 3 (which I suppose is wrong)

Isn't both query snippets are same and suppose to return the same value in my case 0? Why they are producing two different values?

Answer 1

In the second query, you are not counting a column, but a static value for each row in the Products table, you actually are running:

SELECT COUNT('P_Name') FROM DBO.Product

instead of giving the column name in the count:

SELECT COUNT(P_Name) FROM DBO.Product