Difference between two queries 'select statement' with alias table name

Question

I want to ask about the difference between these two queries:

select ProductName = case when p.ProductID is null then 'Unknown' else p.ProductName end

and

select p.ProductName = case when p.ProductID is null then 'Unknown' else p.ProductName end

(p is an alias for table Product)

What is the difference between them in sql server management studio 2008 R2? When I executed the first one, it was executed succesfully, but for the second one I got an error message "Incorrect syntax near '='."

Anyone can explain me about this? Thanks.

Answer 1

in the first case ProductName is not a table column, it is an alias . think of it like this:

select case when p.ProductID is null then 'Unknown' else p.ProductName end as ProductName

in the second incorrect query you are trying to do something illegal because you are trying to both select a column from a table and assign to it as if it were an alias you are generating from an expression.

I think your confusion is that you think ProductName is always a column because there is a column with that name in that table, but in this context ProductName is not a column name; it is an alias for the case statement results. so when you try p.ProductName you are telling the SQL engine to select the ProductName column from the p table and then you try to assign a case statement as if it were an alias (which it isn't in the p.ProductName context).

you could just as easily use any name for the case statement since it is an alias. you could do:

select DerivedProductName = case when p.ProductID is null then 'Unknown' else p.ProductName end

does that example make it more clear that you aren't dealing with column names when you use this syntax?