What IsNull() means in the where clause?-CodePudding

The product table is shown as above.

select * 
from product 
where isnull(product_description, null) = product_description

The SQL script is what I am trying to run.

Can I know how to read or what does it actually mean for the conditions? Why the row 4 is not selected? Feel like the query is same as

select * 
from product 
where product_description is not null

I am a little bit confused about the conditions in the query.

CodePudding user response：

ISNULL takes in two arguments. It returns the first one unless it's null, in which case it returns the second argument - so writing something like ISNULL(SomethingHere, NULL) is meaningless - which means the where clause is the same as where product_description = product_description.

Since in T-SQL NULL = NULL returns UNKNOWN (which is equivalent to false in the context of a WHERE clause), it's basically the same as where product_description is not null.