LINQ Join查询问题答案

【问题标题】：LINQ Join query problemLINQ Join查询问题
【发布时间】：2011-09-28 10:21:00
【问题描述】：

我有这个查询，我尝试将 2 个表连接在一起，其中一个包含产品名称和产品编号，并获取产品编号，然后转到另一个查找与产品编号类似的 Art_no。

ProductNamesList = (From ProductSeries In context.Products
                             From ProductRevisions In context.TestResults
                             Where ((ProductSeries.Product = CurrentProduct) And (ProductRevisions.Art_no.Contains(ProductSeries.Product_no)))
                             Order By ProductRevisions.Art_no
                             Select ProductRevisions.Art_no).Distinct.ToList

希望任何人都可以提供帮助，谢谢。

【问题讨论】：

它似乎没有返回任何值
由于某种原因，它开始工作 O.o 那很奇怪

标签： asp.net vb.net linq

【解决方案1】：

查看帖子：SQL to LINQ ( Case 7 - Filter data by using IN and NOT IN clause)

您的解决方案分为以下两个查询

//first get the list of product which satisfy your condition

    var ids = (from p in context.Products
              Where p.Product = CurrentProduct
              select p.Product_no).ToList();

//second filter the revision products by using contains

    var myProducts = from p in context.TestResults
                     where ids.Contains(p.Art_no)
                     Order By p.Art_no
                     Select p.Art_no;

【讨论】：

ProductSeries.Product_no 是 LIKE ProductRevisions.Art_no，它们不相等
我用 2 个查询来做，试图用 1 个查询得到它

【解决方案2】：

假设类结构如下：

public class Context
{
    public IEnumerable<Product> Products { get; set; }
    public IEnumerable<TestResult> TestResults { get; set; }
}

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class TestResult
{
    public int ArtNo { get; set; }
}

还有一个上下文实例：

var context = new Context
                {
                    Products = new List<Product>
                                {
                                    new Product {Id = 1, Name = "P1"}, 
                                    new Product {Id = 2, Name = "P2"},
                                },
                    TestResults = new List<TestResult>
                                {
                                    new TestResult {ArtNo = 1},
                                    new TestResult {ArtNo = 2}
                                }
                };
var currentProduct = context.Products.ElementAt(0);

这应该会返回您期望的结果：

var query = from product in context.Products
            from testResult in context.TestResults
            where product.Id == testResult.ArtNo
            && currentProduct == product
            orderby testResult.ArtNo
            select product.Name;

// Returns a list with one item "P1"
var productNames = query.Distinct().ToList();

【讨论】：