The LINQ query starts with the anonymous variable groupJoin. Dim groupJoin = ( From order In orders _ĭim line As String = New String( "-", 40)Ĭonsole.WriteLine( "", det.Discount) Listing 1: A group join on the Northwind Traders Orders and Order Details tables. The final listing demonstrates how to get these datum from the Northwind Traders database using LINQ to SQL.) The code demonstrates a group join followed by an array to display the parent and a nested array to display the children of each parent. The fragment in Listing 1 assumes you have a collection of orders and a collection or order details. (The difference between a join-really an inner join-and a group join is that inner joins repeat the parent information for each child.) That is, the child information is coalesced into a collection and the child collection’s parent information occurs only once. The parent information is joined to groups of the child information. Defining a Group JoinĪ group join in LINQ is a join that has an into clause. You also will see some code for LINQ to SQL that is pretty straightforward and my last article, “ Search and Replace with Regular Expressions,” and my upcoming book, LINQ Unleashed: for C#, cover LINQ to SQL in detail. In this article, I will demonstrate the group join because that’s how you get to a left join. (A right join returns orphans but no childless parents. Using another analogy, if the left collection is the parent and the right is the child, a left join is all parents but only children with parents. Usually, the correlation is a key or some kind of unique identifier. A left join is a join whereby all of the elements from the left collection are returned and only elements from the right collection that have a correlated value in the left sequence. One you will call the master or left collection, and the other you’ll call the detail or right collection. Suppose you have two collections of data. The two common joins are the inner join (or just join in LINQ) and the left join. However, left, right, and cross joins are supported by LINQ (with a little nudge). An equijoin uses the join keyword and non-equal joins are contrived using where clauses. As part of the LINQ grammar, you get join and group join. Oddly enough, LINQ doesn’t define keywords for cross join, left join, or right join.
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