Linq Like

Posted 2020-08-21 晓明的哥哥

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：
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：


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 ‘\‘
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。