C# 3.0入門系列（十一）-之In, Like操作

有這麼一個例子，尋找一個表中的某個字段介於某個給定的集合該怎麼辦？Sql寫起來很簡單，比如：Select * from table where id in (2,3, 4, 5)。 就是尋找id字段為這個給定的集合(2,3, 4, 5)內的值。那Linq to Sql該怎麼做呢？一個字，簡單。

In Operator

比如，我們想要查找，"AROUT", "BOLID" 和 "FISSA" 這三個客戶的訂單。該如何做呢？Linq to Sql是這麼做的。

string[] customerID_Set = new string[] { "AROUT", "BOLID", "FISSA" };

var q = (from o in db.Orders
where customerID_Set.Contains(o.CustomerID)
select o).ToList();

其生成的sql語句為

SELECT [t0].[OrderID], [t0].[CustomerID], [t0].[EmployeeID], [t0].[OrderDate], [
t0].[RequiredDate], [t0].[ShippedDate], [t0].[ShipVia], [t0].[Freight], [t0].[Sh
ipName], [t0].[ShipAddress], [t0].[ShipCity], [t0].[ShipRegion], [t0].[ShipPosta
lCode], [t0].[ShipCountry]
FROM [dbo].[Orders] AS [t0]
WHERE [t0].[CustomerID] IN (@p0, @p1, @p2)
-- @p0: Input String (Size = 5; Prec = 0; Scale = 0) [AROUT]
-- @p1: Input String (Size = 5; Prec = 0; Scale = 0) [BOLID]
-- @p2: Input String (Size = 5; Prec = 0; Scale = 0) [FISSA]

先定義了一個數組，在linq query中，使用Contains，也很好理解，就是這個數組，包含了所有的CustomerID, 即返回結果中，所有的CustomerID都在這個集合內。也就是in。 你也可以把數組的定義放在linq語句裡。比如：

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

Not in 呢？加個取反就是

var q2 = (from o in db.Orders
where !(new string[] { "AROUT", "BOLID", "FISSA" }).Contains(o.CustomerID)
select o).ToList();

就這麼簡單。

Like Operator

Like的操作，有點像in，但是，方向變了。什麼意思呢。就是你給定一個字符串，去尋找數據中某個字段包含這個字符串。就是給定的字符串是某字段的子集。Sql Script是這麼寫的。

Selec * from table where id like '%AD%'
Selec * from table where id like '%AD'
Selec * from table where id like 'AD%'

上面的%是通配符，表示，該字段含有某個值，不知道的位置使用%代替。第一個是表示中間一段是AD，兩頭不清楚。第二個是結尾是AD,前面的不清楚。第三個相反，開頭是AD，結尾不清楚。其對應的Linq 語句為

var q = (from c in db.Customers
where c.CustomerID.Contains("ROUT")
select c).ToList();

其生成的sql為

SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactT
itle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Coun
try], [t0].[Phone], [t0].[Fax]
FROM [dbo].[Customers] AS [t0]
WHERE [t0].[CustomerID] LIKE @p0
-- @p0: Input String (Size = 6; Prec = 0; Scale = 0) [%ROUT%]

以ISSA結尾,頭部通配：

var q = (from c in db.Customers
where c.CustomerID.EndsWith("ISSA")
select c).ToList();

其生成的sql為

SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactT
itle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Coun
try], [t0].[Phone], [t0].[Fax]
FROM [dbo].[Customers] AS [t0]
WHERE [t0].[CustomerID] LIKE @p0
-- @p0: Input String (Size = 5; Prec = 0; Scale = 0) [%ISSA]

以ARO開始，尾部通配：

var q = (from c in db.Customers
where c.CustomerID.StartsWith("ARO")
select c).ToList();

其生成的sql為

SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactT
itle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Coun
try], [t0].[Phone], [t0].[Fax]
FROM [dbo].[Customers] AS [t0]
WHERE [t0].[CustomerID] LIKE @p0
-- @p0: Input String (Size = 4; Prec = 0; Scale = 0) [ARO%]

Linq 還提供了一種方法，叫做SqlMethods.Like，需要先添加System.Data.Linq.SqlClient名稱空間。上面的三個可以寫成

var q = (from c in db.Customers
where SqlMethods.Like(c.CustomerID, "%ROUT%")
select c).ToList();

這裡，你需要自己填寫通配符，告訴Linq你是如何匹配。比如

var q = (from c in db.Customers
where SqlMethods.Like(c.CustomerID, "%ISSA")
select c).ToList();

再比如：

var q = (from c in db.Customers
where SqlMethods.Like(c.CustomerID, "ARO%")
select c).ToList();

SqlMethods.Like最奇妙的地方，莫過於，自己定義的通配表達式，你可以在任何地方實現通配。比如

var q = (from c in db.Customers

where SqlMethods.Like(c.CustomerID, "A%O%T")

select c).ToList();

其生成的sql為

SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactT
itle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Coun
try], [t0].[Phone], [t0].[Fax]
FROM [dbo].[Customers] AS [t0]
WHERE [t0].[CustomerID] LIKE @p0
-- @p0: Input String (Size = 5; Prec = 0; Scale = 0) [A%O%T]

就是最標准的知道以A開頭，以T結尾，中間知道一個值O，其他就什麼不知道了。就用這個。

SQL Server 定義了四種通配符，在這裡都可以使用。它們是：

Wildcard character Description Example % Any string of zero or more characters. WHERE title LIKE '%computer%' finds all book titles with the word 'computer' anywhere in the book title. _ (underscore) Any single character. WHERE au_fname LIKE '_ean' finds all four-letter first names that end with ean (Dean, Sean, and so on). [ ] Any single character within the specified range ([a-f]) or set ([abcdef]). WHERE au_lname LIKE '[C-P]arsen' finds author last names ending with arsen and beginning with any single character between C and P, for example Carsen, Larsen, Karsen, and so on. [^] Any single character not within the specified range ([^a-f]) or set ([^abcdef]). WHERE au_lname LIKE 'de[^l]%' all author last names beginning with de and where the following letter is not l. %表示零長度或任意長度的字符串。_表示一個字符。[]表示在某范圍區間的一個字符。[^]表示不在某范圍區間的一個字符

比如：

var q = (from c in db.Customers
where SqlMethods.Like(c.CustomerID, "A_O_T")
select c).ToList();

就用_代表一個字符。其生成sql為

SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactT
itle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Coun
try], [t0].[Phone], [t0].[Fax]
FROM [dbo].[Customers] AS [t0]
WHERE [t0].[CustomerID] LIKE @p0
-- @p0: Input String (Size = 5; Prec = 0; Scale = 0) [A_O_T]

對於Not Like，也很簡單，加個取非就是。

var q = (from c in db.Customers
where !SqlMethods.Like(c.CustomerID, "A_O_T")
select c).ToList();

SqlMethods.Like還有一個參數，叫escape Character，其將會被翻譯成類似下面的語句。

SELECT columns FROM table WHERE
column LIKE '%\%%' ESCAPE '\'

escape 是因為某字段中含有特殊字符，比如%,_ [ ]這些被用作通配符的。這時就要用到Escape了。這是sql server的事情了。詳細情況請參考：

http://msdn2.microsoft.com/en-us/library/Aa933232(SQL.80).aspx