What is a Projection?

Question

What is a Projection, in terms of database theory and NHibernate when using SetProjection()?

Neomineomycin

possible duplicate of What is a Projection in NHibernate? – Surbase

Answer 1

Projection is one of the basic operations of Relational Algebra. It takes a relation and a (possibly empty) list of attributes of that relation as input. It outputs a relation containing only the specified list of attributes with duplicate tuples removed. In other words the output must also be a relation.

Example, if the relation R{A,B}, contains three tuples {1,10},{2,10},{3,20} then the projection of R over the attribute list {B} would contain 2 tuples: {10},{20}.

In short, projection is more or less equivalent to SELECT DISTINCT in SQL (excluding cases with nulls and duplicate columns).

Hyksos

Answer 2

Very simply, it's a function which takes an input (e.g. a database row) and produces an output (e.g. one of the columns from the row, or perhaps some calculation based on multiple columns).

Overtire

In Hibernate, where does the function that produces the output (e.g. the column) get executed? Does it happen on the SQL database server or does it happen on the application/hibernate server after the full row results are returned? Thanks. – Vmail

@KyleM: It's too long since I've used NHibernate to say for sure, but I'd expect it to be executed as part of the SQL. – Overtire

Thanks.. that's what I think as well. I just remembered another way to check (hibernate query logging, which we usually don't use..), so I'll check and report back. – Vmail

@KyleM: The count projection definitely happens in the SQL, if you look at the query that is issued, it is select count(columnName) – Foxhole

Answer 3

Projection means subset of columns in a query.

select x, y, z from YourTable 

x, y, z is the projection here.

Halley

Doesn't it mean "subset of columns" rather than "number of columns"? – Malisamalison

Answer 4

In terms of hibernate, it's like specifying what columns to select. As opposed to letting the mappings determine what columns are fetched. This means you can specify sql functions, subqueries, a single column, or maybe all of the above via a ProjectionList. For example, if you wanted to count the rows in a table SetProjection(Projections.RowCount()).

Overtop

Answer 5

If you are familiar with SQL or database tables: Projection refers to the number of fields/columns/attributes to return. Selection deals with number of rows/records to return. There are good video explanations here and here

Shikoku

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