LINQ標準查詢操作符（二）——Join、GroupJoin、GroupBy、Concat、
阿新 • • 發佈：2019-02-06
四、聯接操作符
聯接是指將一個數據源物件與另一個數據源物件進行關聯或者聯合的操作。這兩個資料來源物件通過一個共同的值或者屬性進行關聯。
LINQ有兩個聯接操作符：Join和GroupJoin。
1. Join
Join操作符類似於T-SQL中的inner join，它將兩個資料來源相聯接，根據兩個資料來源中相等的值進行匹配。例如，可以將產品表與產品類別表相聯接，得到產品名稱和與其相對應的類別名稱。以下的程式碼演示了這一點：
//查詢語法 var query = (from p in db.Products join cin db.Categories on p.CategoryID equals c.CategoryID where p.CategoryID == 1 select new { p.ProductID, p.ProductName, c.CategoryID, c.CategoryName }).ToList(); 生成的sql: SELECT [Extent1].[ProductID] AS [ProductID], [Extent1].[ProductName] AS [ProductName], [Extent2].[CategoryID] AS [CategoryID], [Extent2].[CategoryName] AS [CategoryName] FROM [dbo].[Products] AS [Extent1] INNER JOIN [dbo].[Categories] AS [Extent2] ON [Extent1].[CategoryID]= [Extent2].[CategoryID] WHERE (1 = [Extent1].[CategoryID]) AND ([Extent1].[CategoryID] IS NOT NULL) //方法語法 var q = db.Products .Join ( db.Categories, p => p.CategoryID, c=> c.CategoryID, (p, c) => new { p.ProductID, p.ProductName, c.CategoryID, c.CategoryName } ) .Where(p => p.CategoryID == 1) .ToList(); 生成的sql:
SELECT
[Extent1].[ProductID] AS [ProductID],
[Extent1].[ProductName] AS [ProductName],
[Extent2].[CategoryID] AS [CategoryID],
[Extent2].[CategoryName] AS [CategoryName]
FROM [dbo].[Products] AS [Extent1]
INNER JOIN [dbo].[Categories] AS [Extent2] ON [Extent1].[CategoryID] = [Extent2].[CategoryID]
WHERE 1 = [Extent2].[CategoryID]
以上程式碼為表述清晰加入了一個條件“where p.CategoryID == 1”，即僅返回產品類別ID為1的所有產品。
生成的sql語句略有不同。
2. GroupJoin
GroupJoin操作符常應用於返回“主鍵物件-外來鍵物件集合”形式的查詢，例如“產品類別-此類別下的所有產品”。以下的程式碼演示了這一點：
//查詢語法 var query = (from c in db.Categories join p in db.Products on c.CategoryID equals p.CategoryID into r select new { c.CategoryName, Products = r }).ToList(); //方法語法 var q = db.Categories .GroupJoin ( db.Products, c => c.CategoryID, p => p.CategoryID, (c, p) => new { c.CategoryName, Products = p } ) .ToList(); 生成的sql: SELECT [Project1].[CategoryID] AS [CategoryID], [Project1].[CategoryName] AS [CategoryName], [Project1].[C1] AS [C1], [Project1].[ProductID] AS [ProductID], [Project1].[ProductName] AS [ProductName], [Project1].[SupplierID] AS [SupplierID], [Project1].[CategoryID1] AS [CategoryID1], [Project1].[QuantityPerUnit] AS [QuantityPerUnit], [Project1].[UnitPrice] AS [UnitPrice], [Project1].[UnitsInStock] AS [UnitsInStock], [Project1].[UnitsOnOrder] AS [UnitsOnOrder], [Project1].[ReorderLevel] AS [ReorderLevel], [Project1].[Discontinued] AS [Discontinued] FROM ( SELECT [Extent1].[CategoryID] AS [CategoryID], [Extent1].[CategoryName] AS [CategoryName], [Extent2].[ProductID] AS [ProductID], [Extent2].[ProductName] AS [ProductName], [Extent2].[SupplierID] AS [SupplierID], [Extent2].[CategoryID] AS [CategoryID1], [Extent2].[QuantityPerUnit] AS [QuantityPerUnit], [Extent2].[UnitPrice] AS [UnitPrice], [Extent2].[UnitsInStock] AS [UnitsInStock], [Extent2].[UnitsOnOrder] AS [UnitsOnOrder], [Extent2].[ReorderLevel] AS [ReorderLevel], [Extent2].[Discontinued] AS [Discontinued], CASE WHEN ([Extent2].[ProductID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1] FROM [dbo].[Categories] AS [Extent1] LEFT OUTER JOIN [dbo].[Products] AS [Extent2] ON [Extent1].[CategoryID] = [Extent2].[CategoryID] ) AS [Project1] ORDER BY [Project1].[CategoryID] ASC, [Project1].[C1] ASC
返回的結果為：
五、分組操作符
分組是根據一個特定的值將序列中的元素進行分組。LINQ只包含一個分組操作符：GroupBy。
下面的示例中使用了產品表，以CategoryID作為分組關鍵值，按照產品類別對產品進行了分組。
//查詢語法 var query = (from p in db.Products group p by p.CategoryID).ToList(); //方法語法 var q = db.Products .GroupBy(p => p.CategoryID) .ToList(); 生成的sql: SELECT [Project2].[C1] AS [C1], [Project2].[CategoryID] AS [CategoryID], [Project2].[C2] AS [C2], [Project2].[ProductID] AS [ProductID], [Project2].[ProductName] AS [ProductName], [Project2].[SupplierID] AS [SupplierID], [Project2].[CategoryID1] AS [CategoryID1], [Project2].[QuantityPerUnit] AS [QuantityPerUnit], [Project2].[UnitPrice] AS [UnitPrice], [Project2].[UnitsInStock] AS [UnitsInStock], [Project2].[UnitsOnOrder] AS [UnitsOnOrder], [Project2].[ReorderLevel] AS [ReorderLevel], [Project2].[Discontinued] AS [Discontinued] FROM ( SELECT [Distinct1].[CategoryID] AS [CategoryID], 1 AS [C1], [Extent2].[ProductID] AS [ProductID], [Extent2].[ProductName] AS [ProductName], [Extent2].[SupplierID] AS [SupplierID], [Extent2].[CategoryID] AS [CategoryID1], [Extent2].[QuantityPerUnit] AS [QuantityPerUnit], [Extent2].[UnitPrice] AS [UnitPrice], [Extent2].[UnitsInStock] AS [UnitsInStock], [Extent2].[UnitsOnOrder] AS [UnitsOnOrder], [Extent2].[ReorderLevel] AS [ReorderLevel], [Extent2].[Discontinued] AS [Discontinued], CASE WHEN ([Extent2].[ProductID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C2] FROM (SELECT DISTINCT [Extent1].[CategoryID] AS [CategoryID] FROM [dbo].[Products] AS [Extent1] ) AS [Distinct1] LEFT OUTER JOIN [dbo].[Products] AS [Extent2] ON ([Distinct1].[CategoryID] = [Extent2].[CategoryID]) OR (([Distinct1].[CategoryID] IS NULL) AND ([Extent2].[CategoryID] IS NULL)) ) AS [Project2] ORDER BY [Project2].[CategoryID] ASC, [Project2].[C2] ASC
執行GroupBy得到的序列中包含的元素型別為IGrouping<TKey, T>，其Key屬性代表了分組時使用的關鍵值，遍歷IGrouping<TKey, T>元素可以讀取到每一個T型別。在此示例中，對應的元素型別為IGrouping<int, Products>，其Key屬性即為類別ID，遍歷它可以讀取到每一個產品物件。
六、串聯操作符
串聯是一個將兩個集合聯接在一起的過程。在LINQ中，這個過程通過Concat操作符來實現。
在下面的示例中，將會把類別名稱串聯在產品名稱之後：
//方法語法 var q = db.Products .Select(p => p.ProductName) .Concat ( db.Categories.Select(c => c.CategoryName) ) .ToList(); 生成的sql: SELECT [UnionAll1].[ProductName] AS [C1] FROM (SELECT [Extent1].[ProductName] AS [ProductName] FROM [dbo].[Products] AS [Extent1] UNION ALL SELECT [Extent2].[CategoryName] AS [CategoryName] FROM [dbo].[Categories] AS [Extent2]) AS [UnionAll1]
返回結果77+8=85