C#3.0入门系列(十一)-之In, Like操作
有这么一个例子,寻找一个表中的某个字段介于某个给定的集合该怎么办?Sql写起来很简单,比如:Select * from table where id in (2,3, 4, 5)。 就是寻找id字段为这个给定的集合(2,3, 4, 5)内的值。那Linq to Sql该怎么做呢?一个字,简单。
In Operator
比如,我们想要查找,"AROUT", "BOLID" 和 "FISSA" 这三个客户的订单。该如何做呢?Linq to Sql是这么做的。
string[] customerID_Set = new string[] { "AROUT", "BOLID", "FISSA" };
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var q = (from o in db.Orders
where customerID_Set.Contains(o.CustomerID)
select o).ToList();
其生成的sql语句为
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]
先定义了一个数组,在linq query中,使用Contains,也很好理解,就是这个数组,包含了所有的CustomerID, 即返回结果中,所有的CustomerID都在这个集合内。也就是in。 你也可以把数组的定义放在linq语句里。比如:
var q = (from o in db.Orders
where (new string[] { "AROUT", "BOLID", "FISSA" }).Contains(o.CustomerID)
select o).ToList();
Not in 呢?加个取反就是
var q2 = (from o in db.Orders
where !(new string[] { "AROUT", "BOLID", "FISSA" }).Contains(o.CustomerID)
select o).ToList();
就这么简单。
Like Operator
Like的操作,有点像in,但是,方向变了。什么意思呢。就是你给定一个字符串,去寻找数据中某个字段包含这个字符串。就是给定的字符串是某字段的子集。Sql Script是这么写的。
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,该字段含有某个值,不知道的位置使用%代替。第一个是表示中间一段是AD,两头不清楚。第二个是结尾是AD,前面的不清楚。第三个相反,开头是AD,结尾不清楚。其对应的Linq 语句为
var q = (from c in db.Customers
where c.CustomerID.Contains("ROUT")
select c).ToList();
其生成的sql为
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结尾,头部通配:
var q = (from c in db.Customers
where c.CustomerID.EndsWith("ISSA")
select c).ToList();
其生成的sql为
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开始,尾部通配:
var q = (from c in db.Customers
where c.CustomerID.StartsWith("ARO")
select c).ToList();
其生成的sql为
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 还提供了一种方法,叫做SqlMethods.Like,需要先添加System.Data.Linq.SqlClient名称空间。上面的三个可以写成
var q = (from c in db.Customers
where SqlMethods.Like(c.CustomerID, "%ROUT%")
select c).ToList();
这里,你需要自己填写通配符,告诉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,莫过于,自己定义的通配表达式,你可以在任何地方实现通配。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为
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]
就是最标准的知道以A开头,以T结尾,中间知道一个值O,其他就什么不知道了。就用这个。
SQL Server 定义了四种通配符,在这里都可以使用。它们是:
%表示零长度或任意长度的字符串。_表示一个字符。[]表示在某范围区间的一个字符。[^]表示不在某范围区间的一个字符
比如:
var q = (from c in db.Customers
where SqlMethods.Like(c.CustomerID, "A_O_T")
select c).ToList();
就用_代表一个字符。其生成sql为
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]
对于Not Like,也很简单,加个取非就是。
var q = (from c in db.Customers
where !SqlMethods.Like(c.CustomerID, "A_O_T")
select c).ToList();
SqlMethods.Like还有一个参数,叫escape Character,其将会被翻译成类似下面的语句。
SELECT columns FROM table WHERE
column LIKE '%\%%' ESCAPE '\'
In Operator
比如,我们想要查找,"AROUT", "BOLID" 和 "FISSA" 这三个客户的订单。该如何做呢?Linq to Sql是这么做的。
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Like Operator
Like的操作,有点像in,但是,方向变了。什么意思呢。就是你给定一个字符串,去寻找数据中某个字段包含这个字符串。就是给定的字符串是某字段的子集。Sql Script是这么写的。
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Linq 还提供了一种方法,叫做SqlMethods.Like,需要先添加System.Data.Linq.SqlClient名称空间。上面的三个可以写成
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就是最标准的知道以A开头,以T结尾,中间知道一个值O,其他就什么不知道了。就用这个。
SQL Server 定义了四种通配符,在这里都可以使用。它们是:
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. |
%表示零长度或任意长度的字符串。_表示一个字符。[]表示在某范围区间的一个字符。[^]表示不在某范围区间的一个字符
比如:
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对于Not Like,也很简单,加个取非就是。
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SqlMethods.Like还有一个参数,叫escape Character,其将会被翻译成类似下面的语句。
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escape 是因为某字段中含有特殊字符,比如%,_ [ ]这些被用作通配符的。这时就要用到Escape了。这是sql server的事情了。详细情况请参考:
http://msdn2.microsoft.com/en-us/library/Aa933232(SQL.80).aspx
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