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  • LINQ系列:LINQ to SQL Select查询

    1. 查询全部字段

    using (NorthwindContext context = new NorthwindContext())
    {
        var expr = context.Products;
    
        foreach (var item in expr)
        {
            Console.WriteLine(item.ProductName);
        }
    }
    SELECT 
        [Extent1].[ProductID] AS [ProductID], 
        [Extent1].[CategoryID] AS [CategoryID], 
        [Extent1].[ProductName] AS [ProductName], 
        [Extent1].[UnitPrice] AS [UnitPrice], 
        [Extent1].[Discontinued] AS [Discontinued]
        FROM [dbo].[Product] AS [Extent1]

    2. 查询指定字段

    var expr = context.Products
        .Select(p => new { p.ProductID, p.ProductName });
    var expr = from p in context.Products
                select new { 商品ID = p.ProductID, 商品名称 = p.ProductName };

      上述两种形式所生成的SQL是一样的。

    SELECT 
        [Extent1].[ProductID] AS [ProductID], 
        [Extent1].[ProductName] AS [ProductName]
        FROM [dbo].[Product] AS [Extent1]

    3. First/FirstOrDefault

    var expr = context.Products.First();
    SELECT TOP (1) 
        [c].[ProductID] AS [ProductID], 
        [c].[CategoryID] AS [CategoryID], 
        [c].[ProductName] AS [ProductName], 
        [c].[UnitPrice] AS [UnitPrice], 
        [c].[Discontinued] AS [Discontinued]
        FROM [dbo].[Product] AS [c]

    4. Single/SingleOrDefault

    var expr = context.Products
        .SingleOrDefault(p => p.ProductID == 1);
    SELECT TOP (2) 
        [Extent1].[ProductID] AS [ProductID], 
        [Extent1].[CategoryID] AS [CategoryID], 
        [Extent1].[ProductName] AS [ProductName], 
        [Extent1].[UnitPrice] AS [UnitPrice], 
        [Extent1].[Discontinued] AS [Discontinued]
        FROM [dbo].[Product] AS [Extent1]
        WHERE 1 = [Extent1].[ProductID]
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  • 原文地址:https://www.cnblogs.com/libingql/p/4047609.html
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