What is a Projection?

Question

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

possible duplicate of [What is a Projection in NHibernate?](http://stackoverflow.com/questions/4746995/what-is-a-projection-in-nhibernate) — philipxy, Jul 15 '15 at 08:29

score 51 · Answer 1 · answered Aug 11 '10 at 19:48

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).

score 36 · Answer 2 · answered Aug 11 '10 at 17:28

36

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).

answered Aug 11 '10 at 17:28

Jon Skeet

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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. – KyleM Oct 30 '12 at 20:23
@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. – Jon Skeet Oct 30 '12 at 22:11
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. – KyleM Oct 31 '12 at 00:20
@KyleM: The count projection definitely happens in the SQL, if you look at the query that is issued, it is select `count(columnName)` – mtyson Apr 28 '13 at 19:33

score 21 · Answer 3 · edited Oct 25 '17 at 09:14

21

Projection means subset of columns in a query.

select x, y, z from YourTable

x, y, z is the projection here.

edited Oct 25 '17 at 09:14

Mohammed H

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answered Feb 13 '15 at 13:19

Bilal Anees

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Doesn't it mean "subset of columns" rather than "number of columns"? – CodeManX Mar 31 '16 at 21:26

score 2 · Answer 4 · answered Jul 04 '15 at 19:35

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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

answered Jul 04 '15 at 19:35

Dave

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While these links may answer the question, it is better to include the essential parts of the answer here and provide the links for reference. Link-only answers can become invalid if the linked page changes. – Cleb Jul 04 '15 at 20:02

score 2 · Answer 5 · answered Aug 11 '10 at 19:59

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()).

What is a Projection?

5 Answers5

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