Why is the Entity framework generating this SQL?

Question

I have this LINQ statement,

var carriageways = from carriageway in dataModelCurrentEntities.Carriageway
                   where carriageway.RoadId == roadId &am         


        

Answer 1

Whether or not it produces the SQL with the subquery probably depends on the entity framework provider that you are using. But since most of the existing ones probably share the same pedigree (in that they likely started from a Microsoft code sample), they probably all result in similar SQL. The provider is given a query tree that is produced from the Linq statement and is responsible for producing the SQL. The process for doing this is to visit the nodes in the query tree and generate SQL as it goes.

In the given projection in the OP, it makes sense that the subquery is generated. It is asking for a set of values (new ... {StartMetres, EndMetres}) which are taken from the preceding "query". The query generation would thus produce "SELECT <requested values> FROM something" where the "something" is, itself, rendered as a query. The simple visiting of the query tree, thus, results in a subquery.

Once that process is finished, it would certainly be possible for the provider to "optimize" the resulting SQL and remove the subquery. However, that is the kind of thing that SQL query engines are really good at, so it makes sense to delegate that task to the query engine. It probably depends on the database you are using, but it is likely that the query plan for the SQL statement with the subquery will be identical to the "optimized" one without the subquery.