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CHAPTER 30 LINQ TO ENTITIES
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Figure 30-12. Properties of the Employee LastName field The most important information for us at this point is that we can tell the LastName property is represented using a C# string. When the data model is created, the SQL data types are mapped to C# types. Table 30-2 shows you how some common SQL types are mapped. Table 30-2. SQL to C# Type Mappings
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id int
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bigint char nchar ntext nvarchar text varchar xml date decimal
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CHAPTER 30 LINQ TO ENTITIES
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numeric float real
C# Type
double float
Understanding the IQueryable<T> Interface
We now know that the context Employees property returns an IEnumerable<Employee>, where each Employee represents a row in the table. We also know that an Employee object has properties representing each column in the table. Put these together, and we can understand what the query in Listing 30-1 does: IQueryable<Employee> result = from e in context.Employees where e.City == "London" select e; The data source for the query is the set of data rows in the Employees database table. The where clause filters those rows so that only those whose City property has the value London are included. The select clause projects the Employee object into the results. Notice that the result is an IQueryable<Employee>. This is an interface that is derived from IEnumerable<T> and is used by the query provider to translate LINQ queries into database queries. We don t need to use this interface directly. We can upcast to IEnumerable<T> for consistency with other LINQ queries, but when you see this used in code, you can tell that an external data source will be queried for data. There is one interesting thing that we can do with this interface, and that is to display the SQL query that a LINQ query results in. This can be used to fine-tune the SQL so that we only request the data we need and don t generate unnecessary load on the database server. The technique for getting the SQL string relies on a class that is beyond the scope of this book, so I ll just show you how to achieve it without explaining how it works. Here is the statement that creates a string showing the SQL for Listing 30-1 and prints it to the console:
Console.WriteLine(((System.Data.Objects.ObjectQuery)result).ToTraceString());
You can only do this when you are performing LINQ to Entities queries. Adding this statement to Listing 30-1 shows us what SQL was generated to satisfy the LINQ query: SELECT [Extent1].[EmployeeID] AS [EmployeeID], [Extent1].[LastName] AS [LastName], [Extent1].[FirstName] AS [FirstName], [Extent1].[Title] AS [Title], [Extent1].[TitleOfCourtesy] AS [TitleOfCourtesy], [Extent1].[BirthDate] AS [BirthDate], [Extent1].[HireDate] AS [HireDate], [Extent1].[Address] AS [Address], [Extent1].[City] AS [City],
CHAPTER 30 LINQ TO ENTITIES
[Extent1].[Region] AS [Region], [Extent1].[PostalCode] AS [PostalCode], [Extent1].[Country] AS [Country], [Extent1].[HomePhone] AS [HomePhone], [Extent1].[Extension] AS [Extension], [Extent1].[Photo] AS [Photo], [Extent1].[Notes] AS [Notes], [Extent1].[ReportsTo] AS [ReportsTo], [Extent1].[PhotoPath] AS [PhotoPath] FROM [dbo].[Employees] AS [Extent1] WHERE N'London' = [Extent1].[City] The select clause in the query in Listing 30-1 projects the entire Employee object into the results for the LINQ query, which creates SQL query that retrieves all the columns in the Employees table. Given that we only print out the value of the FirstName and LastName properties, we can reduce the amount of data we request from the database by limiting what we project. Here is a modified query that selects only the data we want: var result = from e in context.Employees where e.City == "London" select new { FirstName = e.FirstName, LastName = e.LastName }; The SQL query generated to satisfy this query is as follows: SELECT 1 AS [C1], [Extent1].[FirstName] AS [FirstName], [Extent1].[LastName] AS [LastName] FROM [dbo].[Employees] AS [Extent1] WHERE N'London' = [Extent1].[City] As a general rule, it is good practice to avoid retrieving data from the database if you don t need to use it in your program. You can see that projecting only the fields we need in our LINQ select clause makes for a much simpler SQL query.
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