This is a conceptual question, please feel free to point me to any tutorials or docs that explain this, but I haven't found them. I can RTFM and read the APIs, but only when I understand the concepts behind the terms.

I am well versed in SQL functionality and DB concepts, so if anyone can explain Alias and Projection in terms of joins, group bys, AS's, and other terms I would appreciate it.

Specifically, I am playing with the Criteria API.

Set Alias seems to be doing some joins and also setting an "AS" to refer to the joined table later. Is the entirety of it?

The Projection concept is a lot bigger and I really dont get it. It seems to be some kind of wrapper that can hold many different types of values to be held later. Those types of values are often aggregate values and/or group-by clauses, but can also be arbitrary SQL statements. Huh?

Thanks again!

With projection you can do the things you can do in the select part of an SQL query.

Alias is like the AS statement in SQL but it may force a join in some cases (as far as I know).

More specifically... what does that mean?



It seems like sometimes it does and sometimes it doesn't which is why I am trying to understand conceptually what an Alias is, so that I can understand better.

projections basically define what columns will be returned whether they are a property or possibly the result of an aggregate function like sum() or avg()

the example in the docs

List results = session.createCriteria(Cat.class)
.setProjection( Property.forName("name") )
.add( Property.forName("color").eq(Color.BLACK) )
.list();

I believe would return one column "name" with names of all cats with color == black.

I have read the docs. Here is an example of a question I have that i cant find covered by the docs, which leads me to try and understand the meaning of "Projection":

How can i do an Order by of a Grouping expression when i dont want to return the grouping?

in HQL: select cat from Cat cat order by size(cat.kittens) desc

Can anyone explain to me how to express that in Criteria API?

Trying: criteria.addOrder(Order.desc("size(kittens)");
throws:
org.hibernate.QueryException: could not resolve property: size(kittens) of: ..Cat

So I assume I have to use Projections for the aggregate size, but I dont want it in my select statement.

Or, what if instead of "size(..)" being an aggregate query, it was a DB function like "FLOOR(..)"? Would that also use a Projection? (a SQLProjection?)