将 T-sql 查询转换为 linq答案

【问题标题】：Converting T-sql query into linq将 T-sql 查询转换为 linq
【发布时间】：2017-03-04 20:01:03
【问题描述】：

我要翻译的查询是：

SELECT
    Products.id, [Products].Name 
FROM 
    [Products] 
LEFT JOIN 
    [AvailableProducts] ON [AvailableProducts].IdProduct = [Products].Id
                        AND [AvailableProducts].IdUser = 'daa8ee71-f1d7-4b41-aa1c-464ae730dd63' 
WHERE 
   ([AvailableProducts].IdProduct IS NULL)

我正在尝试用这个 Linq 代码解决这个问题：

public void DataBindGridProducts(string idUser)
{
     List<Product> data = new List<Product>();

     var con = from test in db.AvailableProducts 
               join product in db.Products on new { test.IdProduct, test.IdUser} equals new { product.Id, idUser} 
               select new
                       {
                           surname = product.Name,
                       };

     grdConnectedProduct.DataSource = data;
     grdConnectedProduct.DataBind();
}

但这是不正确的。请帮帮我！

【问题讨论】：

标签： sql asp.net entity-framework linq tsql

【解决方案1】：

public void DataBindGridProducts(string idUser)
{
    List<Product> data = new List<Product>();

    var con = from product in db.Products
              join ap in db.AvailableProducts on new { x= (int)product.Id, y = product.idUser } equals new { x = (int)ap.IdProduct, y = ap.IdUser }
              where product.IdProduct = null
              select new
                      {
                          surname = product.Name,
                      };

    grdConnectedProduct.DataSource = data;
    grdConnectedProduct.DataBind();
}

而你使用 lambda 表达式。

我认为下面的代码应该可以工作

public void DataBindGridProducts(string idUser)
{
    List<Product> data = new List<Product>();

    var con = db.Products.Join(db.AvailableProducts,
                               product => new{(int)product.Id, product.idUser},
                               ap => new { (int)ap.IdProduct, ap.IdUser },
                               (product, ap) => product)
                         .Where(product => product.IdProduct = null)
                         .Select(x => x new
                                        {
                                              surname = x.Name,
                                        })
                         .ToList();

    grdConnectedProduct.DataSource = data;
    grdConnectedProduct.DataBind();
}

【讨论】：

【解决方案2】：

你的 t-sql 查询等于这个查询：

SELECT
    [product].[Id],
    [product].[Name]
FROM
    [Products] [product]
        LEFT JOIN (
            SELECT
                [j].[IdProduct],
                [j].[IdUser]
            FROM
                [AvailableProducts] [j]
            WHERE
                [j].[IdUser] = N'daa8ee71-f1d7-4b41-aa1c-464ae730dd63'
        ) [t1] ON [product].[Id] = [t1].[IdProduct]
WHERE
    [t1].[IdProduct] IS NULL AND [t1].[IdUser] IS NULL

而 Linq 查询有这样的观点：

var availableProductsQuery = from ap in db.AvailableProducts
                                             where ap.IdUser == "daa8ee71-f1d7-4b41-aa1c-464ae730dd63"
                                             select ap;

                var con = from product in db.Products.AsQueryable()
                          join test in availableProductsQuery
                            on product.Id equals test.IdProduct into j
                          from ap in j.DefaultIfEmpty()
                          where ap == null
                          select product;

【讨论】：

【解决方案3】：

我修好了：

var o = (from p in db.Products
                join p2 in db.AvailableProducts on new { f1 = (int)p.Id, f2 = idUser } equals new { f1 = (int)p2.IdProduct, f2 = p2.IdUser }

                into temp
                from x in temp.DefaultIfEmpty()
                 where x.IdProduct == null
                 select new { f1 = p.Name});

【讨论】：