上文寫了關于Where及Select的基本操作，這一篇講一下Where的進階操作

先說一下查看所生成的SQL的方法，.ToTraceString()
例如：

string entitySQL = "select value a from orders as a;";
var query = db.CreateQuery<Orders>(entitySQL);
Console.WriteLine(query.ToTraceString());

參數

參數可以通過CreateQuery的第二個參數傳遞進去。

DateTime dt = new DateTime(1997, 1, 1);
ObjectParameter[] ps = { new ObjectParameter("dt", dt) };
ObjectQuery<Orders> query = db.CreateQuery<Orders>("SELECT VALUE Orders FROM Orders WHERE Orders.OrderDate < @dt;", ps);

這里通過@dt將參數傳了進去，執行結果：

外鍵關系

Entity SQL與其它的查詢一樣，可以通過外鍵的關系直接取值或判斷，如：

ObjectQuery<Orders> query = db.CreateQuery<Orders>(
"SELECT VALUE o FROM Orders AS o WHERE o.Customers.Country = 'Mexico';");

?

我們直接用o.Customers.Country這是一個一對多的外鍵關系。

多條件查詢

與T-SQL相同，只要用相應的邏輯關鍵字即可。如：

string entitySQL =
"SELECT VALUE p FROM Products AS p WHERE p.UnitsInStock < p.ReorderLevel and p.UnitsOnOrder = 0;";
ObjectQuery<Products> query = db.CreateQuery<Products>(entitySQL);

Exists查詢

做為Where的一個經常用的謂詞，Exists的使用方法如下：

string entitySQL = @"SELECT VALUE s FROM Suppliers AS s WHERE
EXISTS(SELECT p FROM s.Products AS p WHERE p.UnitsInStock = 0);";
var query = db.CreateQuery<bool>(entitySQL);

運行后生成的SQL如下：

SELECT
[Extent1].[Address] AS [Address],
[Extent1].[City] AS [City],
[Extent1].[CompanyName] AS [CompanyName],
[Extent1].[ContactName] AS [ContactName],
[Extent1].[ContactTitle] AS [ContactTitle],
[Extent1].[Country] AS [Country],
[Extent1].[Fax] AS [Fax],
[Extent1].[HomePage] AS [HomePage],
[Extent1].[Phone] AS [Phone],
[Extent1].[PostalCode] AS [PostalCode],
[Extent1].[Region] AS [Region],
[Extent1].[SupplierID] AS [SupplierID]
FROM [dbo].[Suppliers] AS [Extent1]
WHERE EXISTS (SELECT
cast(1 as bit) AS [C1]
FROM [dbo].[Products] AS [Extent2]
WHERE ([Extent1].[SupplierID] = [Extent2].[SupplierID]) AND ([Extent2].[
UnitsInStock] = 0)
)

IN查詢

In查詢當然也是不可少的。使用方法如下：

string entitySQL =
@"SELECT VALUE c FROM Customers AS c WHERE c.City in {'Seattle','harbin'}";
var query = db.CreateQuery<Orders>(entitySQL);

LIKE查詢

like做為一個模糊查詢的關鍵字是一定要支持的。

string entitySQL =
@"SELECT VALUE c FROM Customers AS c WHERE c.City like '%at%';";
var query = db.CreateQuery<Orders>(entitySQL);

Between、And查詢

string entitySQL =@"SELECT VALUE c FROM Customers AS c WHERE c.ID between 1 and 100";
var query = db.CreateQuery<Orders>(entitySQL);

總結

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