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]