LINQ to SQL語句(7)之Exists/In/Any/All/Contains

適用場景：用於判斷集合中元素，進一步縮小範圍。

Any

說明：用於判斷集合中是否有元素知足某一條件；不延遲。（若條件爲空，則集合只要不爲空就返回True，不然爲False）。有2種形式，分別爲簡單形式和帶條件形式。

1.簡單形式：

僅返回沒有訂單的客戶：

var q =
    from c in db.Customers
    where !c.Orders.Any()
    select c;

生成SQL語句爲：

SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName],
[t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region],
[t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax]
FROM [dbo].[Customers] AS [t0]
WHERE NOT (EXISTS(
    SELECT NULL AS [EMPTY] FROM [dbo].[Orders] AS [t1]
    WHERE [t1].[CustomerID] = [t0].[CustomerID]
   ))

2.帶條件形式：

僅返回至少有一種產品斷貨的類別：

var q =
    from c in db.Categories
    where c.Products.Any(p => p.Discontinued)
    select c;

SELECT [t0].[CategoryID], [t0].[CategoryName], [t0].[Description],
[t0].[Picture] FROM [dbo].[Categories] AS [t0]
WHERE EXISTS(
    SELECT NULL AS [EMPTY] FROM [dbo].[Products] AS [t1]
    WHERE ([t1].[Discontinued] = 1) AND 
    ([t1].[CategoryID] = [t0].[CategoryID])
    )

All

說明：用於判斷集合中全部元素是否都知足某一條件；不延遲

1.帶條件形式

var q =
    from c in db.Customers
    where c.Orders.All(o => o.ShipCity == c.City)
    select c;

語句描述：這個例子返回全部訂單都運往其所在城市的客戶或未下訂單的客戶。

Contains

說明：用於判斷集合中是否包含有某一元素；不延遲。它是對兩個序列進行鏈接操做的。

string[] customerID_Set =
    new string[] { "AROUT", "BOLID", "FISSA" };
var q = (
    from o in db.Orders
    where customerID_Set.Contains(o.CustomerID)
    select o).ToList();

語句描述：查找"AROUT", "BOLID" 和 "FISSA" 這三個客戶的訂單。先定義了一個數組，在LINQ to SQL中使用Contains，數組中包含了全部的CustomerID，即返回結果中，全部的CustomerID都在這個集合內。也就是in。 你也能夠把數組的定義放在LINQ to SQL語句裏。好比：

var q = (
    from o in db.Orders
    where (
    new string[] { "AROUT", "BOLID", "FISSA" })
    .Contains(o.CustomerID)
    select o).ToList();

Not Contains則取反：

var q = (
    from o in db.Orders
    where !(
    new string[] { "AROUT", "BOLID", "FISSA" })
    .Contains(o.CustomerID)
    select o).ToList();

1.包含一個對象：

var order = (from o in db.Orders
             where o.OrderID == 10248
             select o).First();
var q = db.Customers.Where(p => p.Orders.Contains(order)).ToList();
foreach (var cust in q)
{
    foreach (var ord in cust.Orders)
    {
        //do something
    }
}

語句描述：這個例子使用Contain查找哪一個客戶包含OrderID爲10248的訂單。

2.包含多個值：

string[] cities = 
    new string[] { "Seattle", "London", "Vancouver", "Paris" };
var q = db.Customers.Where(p=>cities.Contains(p.City)).ToList();

語句描述：這個例子使用Contains查找其所在城市爲西雅圖、倫敦、巴黎或溫哥華的客戶。