LINQ to SQL.

http://weblogs.asp.net/rajbk/archive/2010/03.aspx

 

The following post shows how to write different types of joins in LINQ to SQL. I am using the Northwind database and LINQ to SQL for these examples.

NorthwindDataContext dataContext = new NorthwindDataContext();

Inner Join

var q1 = from c in dataContext.Customers

         join o in dataContext.Orders on c.CustomerID equals o.CustomerID

         select new

         {

             c.CustomerID,

             c.ContactName,

             o.OrderID,

             o.OrderDate

         };

SELECT [t0].[CustomerID], [t0].[ContactName], [t1].[OrderID], [t1].[OrderDate]

FROM [dbo].[Customers] AS [t0]

INNER JOIN [dbo].[Orders] AS [t1] ON [t0].[CustomerID] = [t1].[CustomerID]

Left Join

var q2 = from c in dataContext.Customers

         join o in dataContext.Orders on c.CustomerID equals o.CustomerID into g

         from a in g.DefaultIfEmpty()

         select new

         {

             c.CustomerID,

             c.ContactName,

             a.OrderID,

             a.OrderDate

         };

SELECT [t0].[CustomerID], [t0].[ContactName], [t1].[OrderID] AS [OrderID], [t1].[OrderDate] AS [OrderDate]

FROM [dbo].[Customers] AS [t0]

LEFT OUTER JOIN [dbo].[Orders] AS [t1] ON [t0].[CustomerID] = [t1].[CustomerID]

Inner Join on multiple

//We mark our anonymous type properties as a and b otherwise

//we get the compiler error "Type inferencce failed in the call to 'Join’

var q3 = from c in dataContext.Customers

         join o in dataContext.Orders on new { a = c.CustomerID, b = c.Country } equals new { a = o.CustomerID, b = "USA" }

         select new

         {

             c.CustomerID,

             c.ContactName,

             o.OrderID,

             o.OrderDate

         };

SELECT [t0].[CustomerID], [t0].[ContactName], [t1].[OrderID], [t1].[OrderDate]

FROM [dbo].[Customers] AS [t0]

INNER JOIN [dbo].[Orders] AS [t1] ON ([t0].[CustomerID] = [t1].[CustomerID]) AND ([t0].[Country] = @p0)

Inner Join on multiple with ‘OR’ clause

var q4 = from c in dataContext.Customers

         from o in dataContext.Orders.Where(a => a.CustomerID == c.CustomerID || c.Country == "USA")

         select new

         {

             c.CustomerID,

             c.ContactName,

             o.OrderID,

             o.OrderDate

         };

SELECT [t0].[CustomerID], [t0].[ContactName], [t1].[OrderID], [t1].[OrderDate]

FROM [dbo].[Customers] AS [t0], [dbo].[Orders] AS [t1]

WHERE ([t1].[CustomerID] = [t0].[CustomerID]) OR ([t0].[Country] = @p0)

Left Join on multiple with ‘OR’ clause

var q5 = from c in dataContext.Customers

         from o in dataContext.Orders.Where(a => a.CustomerID == c.CustomerID || c.Country == "USA").DefaultIfEmpty()

         select new

         {

             c.CustomerID,

             c.ContactName,

             o.OrderID,

             o.OrderDate

         };

SELECT [t0].[CustomerID], [t0].[ContactName], [t1].[OrderID] AS [OrderID], [t1].[OrderDate] AS [OrderDate]

FROM [dbo].[Customers] AS [t0]

LEFT OUTER JOIN [dbo].[Orders] AS [t1] ON ([t1].[CustomerID] = [t0].[CustomerID]) OR ([t0].[Country] = @p0)