LINQ to SQL Queries
You define LINQ to SQL queries by using the same syntax as you would in LINQ. The only difference is that the objects referenced in your queries are mapped to elements in a database. For more information, see Introduction to LINQ Queries (C#).
LINQ to SQL translates the queries you write into equivalent SQL queries and sends them to the server for processing. More specifically, your application uses the LINQ to SQL API to request query execution. The LINQ to SQL provider then transforms the query into SQL text and delegates execution to the ADO provider. The ADO provider returns query results as a DataReader. The LINQ to SQL provider translates the ADO results to an IQueryable collection of user objects.
Note
Most methods and operators on .NET Framework built-in types have direct translations to SQL. Those that LINQ cannot translate generate run-time exceptions. For more information, see SQL-CLR Type Mapping.
The following table shows the similarities and differences between LINQ and LINQ to SQL query items.
| Item | LINQ Query | LINQ to SQL Query |
|---|---|---|
| Return type of the local variable that holds the query (for queries that return sequences) | Generic IEnumerable |
Generic IQueryable |
| Specifying the data source | Uses the From (Visual Basic) or from (C#) clause |
Same |
| Filtering | Uses the Where/where clause |
Same |
| Grouping | Uses the Group…By/groupby clause |
Same |
| Selecting (Projecting) | Uses the Select/select clause |
Same |
| Deferred versus immediate execution | See Introduction to LINQ Queries (C#) | Same |
| Implementing joins | Uses the Join/join clause |
Can use the Join/join clause, but more effectively uses the AssociationAttribute attribute. For more information, see Querying Across Relationships. |
| Remote versus local execution | For more information, see Remote vs. Local Execution. | |
| Streaming versus cached querying | Not applicable in a local memory scenario |