Entity-framework – “SELECT VALUE” – value keyword in LINQ/Entity Framework query

entity-frameworklinqlinq-to-entities

What does the keyword "value" mean in this statement, and where would I go to learn more?
What happens if I leave out the keyword "value"? In the code below, z is an entity framework class.

string queryString = "SELECT VALUE q from x.zs as q where q.a = @parm;"
ObjectQuery<z> query = context.CreateQuery<z> 
    (queryString, new ObjectParameter("parmname",parmvalue)); 
return query.First();

(This is a part of a practice question for an exam).

The above code is in a function that returns a variable of type z.

Best Answer

That is Entity SQL syntax. Value keyword allows only one value to be specified, and does not add a row wrapper.

Read article about SELECT statement in ESQL

Entity SQL supports two variants of the SELECT clause. The first variant, row select, is identified by the SELECT keyword, and can be used to specify one or more values that should be projected out. Because a row wrapper is implicitly added around the values returned, the result of the query expression is always a multiset of rows.

Each query expression in a row select must specify an alias. If no alias is specified,Entity SQL attempts to generate an alias by using the alias generation rules.

The other variant of the SELECT clause, value select, is identified by the SELECT VALUE keyword. It allows only one value to be specified, and does not add a row wrapper.

So, if you want to materialize z object from your query, you should use SELECT VALUE syntax (otherwise you will get exception: cast from MaterializedDataRecord to z type is not valid).

Without VALUE keyword you will get set of rows:

string esql = "SELECT q from x.zs as q where q.a = @parm;";
ObjectQuery<DbDataRecord> query = context
       .CreateQuery<DbDataRecord>(esql, new ObjectParameter("parm",parmvalue)); 
var result = query.First();

Related Solutions

.net – Entity Framework vs LINQ to SQL

LINQ to SQL only supports 1 to 1 mapping of database tables, views, sprocs and functions available in Microsoft SQL Server. It's a great API to use for quick data access construction to relatively well designed SQL Server databases. LINQ2SQL was first released with C# 3.0 and .Net Framework 3.5.

LINQ to Entities (ADO.Net Entity Framework) is an ORM (Object Relational Mapper) API which allows for a broad definition of object domain models and their relationships to many different ADO.Net data providers. As such, you can mix and match a number of different database vendors, application servers or protocols to design an aggregated mash-up of objects which are constructed from a variety of tables, sources, services, etc. ADO.Net Framework was released with the .Net Framework 3.5 SP1.

This is a good introductory article on MSDN: Introducing LINQ to Relational Data

C# – LINQ query on a DataTable

You can't query against the DataTable's Rows collection, since DataRowCollection doesn't implement IEnumerable<T>. You need to use the AsEnumerable() extension for DataTable. Like so:

var results = from myRow in myDataTable.AsEnumerable()
where myRow.Field<int>("RowNo") == 1
select myRow;

And as @Keith says, you'll need to add a reference to System.Data.DataSetExtensions

AsEnumerable() returns IEnumerable<DataRow>. If you need to convert IEnumerable<DataRow> to a DataTable, use the CopyToDataTable() extension.

Below is query with Lambda Expression,

var result = myDataTable
    .AsEnumerable()
    .Where(myRow => myRow.Field<int>("RowNo") == 1);

Best Answer

Related Solutions

.net – Entity Framework vs LINQ to SQL

C# – LINQ query on a DataTable

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