LINQ系列：LINQ to SQL Join连接

最新推荐文章于 2021-08-29 13:33:40 发布

转载最新推荐文章于 2021-08-29 13:33:40 发布 · 183 阅读

本文深入探讨了SQL中的一对多查询、双向关联、左外连接和内联多个表的高级查询技巧，通过具体代码实例展示如何进行复杂的数据关联与筛选。

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1. 一对多

var expr = context.Products
    .Where(p => p.Category.CategoryName == "LINQ to SQL" && p.UnitPrice > 10m)
    .Select(p => new
    {
        p.ProductID,
        p.ProductName
    });

var expr = from p in context.Products
            where p.Category.CategoryName == "LINQ to SQL" && p.UnitPrice > 10m
            select new
            {
                p.ProductID,
                p.ProductName
            };

SELECT 
    [Extent1].[ProductID] AS [ProductID], 
    [Extent1].[ProductName] AS [ProductName]
    FROM  [dbo].[Product] AS [Extent1]
    INNER JOIN [dbo].[Category] AS [Extent2] ON [Extent1].[CategoryID] = [Extent2].[CategoryID]
    WHERE (N'LINQ to SQL' = [Extent2].[CategoryName]) AND ([Extent1].[UnitPrice] > cast(10 as decimal(18)))

var expr = from p in context.Products
            where p.UnitPrice > 10m
            join c in context.Categories on p.CategoryID equals c.CategoryID
            select new
            {
                p.ProductID,
                p.ProductName,
                c.CategoryName
            };

SELECT 
    [Extent1].[ProductID] AS [ProductID], 
    [Extent1].[ProductName] AS [ProductName], 
    [Extent2].[CategoryName] AS [CategoryName]
    FROM  [dbo].[Product] AS [Extent1]
    INNER JOIN [dbo].[Category] AS [Extent2] ON [Extent1].[CategoryID] = [Extent2].[CategoryID]
    WHERE [Extent1].[UnitPrice] > cast(10 as decimal(18))

var expr = from p in context.Products
           join c in context.Categories on p.CategoryID equals c.CategoryID
           select new
           {
               p.ProductID,
               p.ProductName,
               c.CategoryName
           };

SELECT 
    [Extent1].[ProductID] AS [ProductID], 
    [Extent1].[ProductName] AS [ProductName], 
    [Extent2].[CategoryName] AS [CategoryName]
    FROM  [dbo].[Product] AS [Extent1]
    INNER JOIN [dbo].[Category] AS [Extent2] ON [Extent1].[CategoryID] = [Extent2].[CategoryID]

2. 双向关联

var expr = from c in context.Categories
            join p in context.Products on c.CategoryID equals p.CategoryID into cp
            select new
            {
                c.CategoryName,
                TotalCategoryProducts = cp.Count()
            };

SELECT 
    [Extent1].[CategoryID] AS [CategoryID], 
    [Extent1].[CategoryName] AS [CategoryName], 
    (SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[Product] AS [Extent2]
        WHERE [Extent1].[CategoryID] = [Extent2].[CategoryID]) AS [C1]
    FROM [dbo].[Category] AS [Extent1]

3. left out join

var expr = from c in context.Categories
            join p in context.Products on c.CategoryID equals p.CategoryID into CategoryProducts
            from cp in CategoryProducts.DefaultIfEmpty()
            select new
            {
                c.CategoryID,
                c.CategoryName,
                TotalProducts = CategoryProducts.Count()
            };

SELECT 
    [Extent1].[CategoryID] AS [CategoryID], 
    [Extent1].[CategoryName] AS [CategoryName], 
    (SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[Product] AS [Extent3]
        WHERE [Extent1].[CategoryID] = [Extent3].[CategoryID]) AS [C1]
    FROM  [dbo].[Category] AS [Extent1]
    LEFT OUTER JOIN [dbo].[Product] AS [Extent2] ON [Extent1].[CategoryID] = [Extent2].[CategoryID]

4. inner join select many

var expr = from c in context.Categories
           from p in c.Products
           where c.CategoryName == "LINQ to SQL"
           select p;

var expr = context.Categories
                .Where(c => c.CategoryName == "LINQ to SQL")
                .SelectMany(c => c.Products);

SELECT 
    [Extent2].[ProductID] AS [ProductID], 
    [Extent2].[CategoryID] AS [CategoryID], 
    [Extent2].[ProductName] AS [ProductName], 
    [Extent2].[UnitPrice] AS [UnitPrice], 
    [Extent2].[UnitsInStock] AS [UnitsInStock], 
    [Extent2].[Discontinued] AS [Discontinued]
    FROM  [dbo].[Category] AS [Extent1]
    INNER JOIN [dbo].[Product] AS [Extent2] ON [Extent1].[CategoryID] = [Extent2].[CategoryID]
    WHERE N'LINQ to SQL' = [Extent1].[CategoryName]