LINQ系列：LINQ to SQL Transact-SQL函数

1. CASE WHEN ... THEN ...

var expr = from p in context.Products
            select new 
            { 
                商品ID = p.ProductID, 
                商品名称 = p.ProductName, 
                是否库存 = p.UnitsInStock > 0 ? "是" : "否" 
            };

SELECT 
    [Extent1].[ProductID] AS [ProductID], 
    [Extent1].[ProductName] AS [ProductName], 
    CASE WHEN ([Extent1].[UnitsInStock] > 0) THEN N'是' ELSE N'否' END AS [C1]
    FROM [dbo].[Product] AS [Extent1]

2. Distinct

var expr = context.Products
    .Select(p => p.CategoryID)
    .Distinct();

SELECT 
    [Distinct1].[CategoryID] AS [CategoryID]
    FROM ( SELECT DISTINCT 
        [Extent1].[CategoryID] AS [CategoryID]
        FROM [dbo].[Product] AS [Extent1]
    )  AS [Distinct1]

3. Count

var expr = context.Products.Count();

SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[Product] AS [Extent1]
    )  AS [GroupBy1]

var expr = context.Products
    .Count(p => p.UnitPrice > 10m);

SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[Product] AS [Extent1]
        WHERE [Extent1].[UnitPrice] > cast(10 as decimal(18))
    )  AS [GroupBy1]

4. LongCount

var expr = context.Products.LongCount();

SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        COUNT_BIG(1) AS [A1]
        FROM [dbo].[Product] AS [Extent1]
    )  AS [GroupBy1]

var expr = context.Products
    .LongCount(p => p.UnitPrice > 10m);

SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        COUNT_BIG(1) AS [A1]
        FROM [dbo].[Product] AS [Extent1]
        WHERE [Extent1].[UnitPrice] > cast(10 as decimal(18))
    )  AS [GroupBy1]

5. Sum

var expr = context.Products
    .Select(p=>p.UnitsInStock)
    .Sum();

var expr = context.Products
    .Sum(p => p.UnitsInStock);

SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        SUM([Extent1].[UnitsInStock]) AS [A1]
        FROM [dbo].[Product] AS [Extent1]
    )  AS [GroupBy1]

6. Min

var expr = context.Products
    .Min(p => p.UnitPrice);

var expr = context.Products
    .Select(p => p.UnitPrice)
    .Min();

SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        MIN([Extent1].[UnitPrice]) AS [A1]
        FROM [dbo].[Product] AS [Extent1]
    )  AS [GroupBy1]

　　查找每个类别中单价最低的商品：

var expr = from p in context.Products
            group p by p.CategoryID into g
            select new
            {
                CategoryID = g.Key,
                CheapestProducts = from p2 in g
                                    where p2.UnitPrice == g.Min(p3 => p3.UnitPrice)
                                    select p2
            };

SELECT 
    [Project1].[CategoryID] AS [CategoryID], 
    [Project1].[C1] AS [C1], 
    [Project1].[ProductID] AS [ProductID], 
    [Project1].[CategoryID1] AS [CategoryID1], 
    [Project1].[ProductName] AS [ProductName], 
    [Project1].[UnitPrice] AS [UnitPrice], 
    [Project1].[UnitsInStock] AS [UnitsInStock], 
    [Project1].[Discontinued] AS [Discontinued]
    FROM ( SELECT 
        [GroupBy1].[K1] AS [CategoryID], 
        [Extent2].[ProductID] AS [ProductID], 
        [Extent2].[CategoryID] AS [CategoryID1], 
        [Extent2].[ProductName] AS [ProductName], 
        [Extent2].[UnitPrice] AS [UnitPrice], 
        [Extent2].[UnitsInStock] AS [UnitsInStock], 
        [Extent2].[Discontinued] AS [Discontinued], 
        CASE WHEN ([Extent2].[ProductID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]
        FROM   (SELECT 
            [Extent1].[CategoryID] AS [K1], 
            MIN([Extent1].[UnitPrice]) AS [A1]
            FROM [dbo].[Product] AS [Extent1]
            GROUP BY [Extent1].[CategoryID] ) AS [GroupBy1]
        LEFT OUTER JOIN [dbo].[Product] AS [Extent2] ON ([GroupBy1].[K1] = [Extent2].[CategoryID]) AND ([Extent2].[UnitPrice] = [GroupBy1].[A1])
    )  AS [Project1]
    ORDER BY [Project1].[CategoryID] ASC, [Project1].[C1] ASC

7. Max

var expr = context.Products
    .Max(p => p.UnitPrice);

var expr = context.Products
    .Select(p => p.UnitPrice)
    .Max();

SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        MAX([Extent1].[UnitPrice]) AS [A1]
        FROM [dbo].[Product] AS [Extent1]
    )  AS [GroupBy1]

8.Avg

var expr = context.Products
    .Select(p => p.UnitPrice)
    .Average();

var expr = context.Products
    .Average(p => p.UnitPrice);

SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        AVG([Extent1].[UnitPrice]) AS [A1]
        FROM [dbo].[Product] AS [Extent1]
    )  AS [GroupBy1]

　　查找单个类别中单价高于平均价的产品：

var expr = from p in context.Products
            group p by p.CategoryID into g
            select new
            {
                CategoryID = g.Key,
                ExpensiveProducts = from p2 in g
                                    where p2.UnitPrice > g.Average(p3 => p3.UnitPrice)
                                    select p2
            };

SELECT 
    [Project1].[CategoryID] AS [CategoryID], 
    [Project1].[C1] AS [C1], 
    [Project1].[ProductID] AS [ProductID], 
    [Project1].[CategoryID1] AS [CategoryID1], 
    [Project1].[ProductName] AS [ProductName], 
    [Project1].[UnitPrice] AS [UnitPrice], 
    [Project1].[UnitsInStock] AS [UnitsInStock], 
    [Project1].[Discontinued] AS [Discontinued]
    FROM ( SELECT 
        [GroupBy1].[K1] AS [CategoryID], 
        [Extent2].[ProductID] AS [ProductID], 
        [Extent2].[CategoryID] AS [CategoryID1], 
        [Extent2].[ProductName] AS [ProductName], 
        [Extent2].[UnitPrice] AS [UnitPrice], 
        [Extent2].[UnitsInStock] AS [UnitsInStock], 
        [Extent2].[Discontinued] AS [Discontinued], 
        CASE WHEN ([Extent2].[ProductID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]
        FROM   (SELECT 
            [Extent1].[CategoryID] AS [K1], 
            AVG([Extent1].[UnitPrice]) AS [A1]
            FROM [dbo].[Product] AS [Extent1]
            GROUP BY [Extent1].[CategoryID] ) AS [GroupBy1]
        LEFT OUTER JOIN [dbo].[Product] AS [Extent2] ON ([GroupBy1].[K1] = [Extent2].[CategoryID]) AND ([Extent2].[UnitPrice] > [GroupBy1].[A1])
    )  AS [Project1]
    ORDER BY [Project1].[CategoryID] ASC, [Project1].[C1] ASC