Linq to SQL Like Operator(转)

http://www.cnblogs.com/freeliver54/archive/2009/09/05/1560815.html

本文转自：http://blogs.microsoft.co.il/blogs/bursteg/archive/2007/10/16/linq-to-sql-like-operator.aspx
原文如下：

As a response for customer's question, I decided to write about using Like Operator in Linq to SQL queries.

Starting from a simple query from Northwind Database;

var query = from c in ctx.Customers

where c.City == "London"

select c;

The query that will be sent to the database will be:

SELECT CustomerID, CompanyName, ...
FROM dbo.Customers
WHERE City = [London]

There are some ways to write a Linq query that reaults in using Like Operator in the SQL statement:

1. Using String.StartsWith or String.Endswith

Writing the following query:

var query = from c in ctx.Customers

where c.City.StartsWith("Lo")

select c;

will generate this SQL statement:

SELECT CustomerID, CompanyName, ...
FROM dbo.Customers
WHERE City LIKE [Lo%]

which is exactly what we wanted. Same goes with String.EndsWith.

But, what is we want to query the customer with city name like "L_n%"? (starts with a Capital 'L', than some character, than 'n' and than the rest of the name). Using the query

var query = from c in ctx.Customers

where c.City.StartsWith("L") && c.City.Contains("n")

select c;

generates the statement:

SELECT CustomerID, CompanyName, ...
FROM dbo.Customers
WHERE City LIKE [L%]
AND City LIKE [%n%]

which is not exactly what we wanted, and a little more complicated as well.

2. Using SqlMethods.Like method

Digging into System.Data.Linq.SqlClient namespace, I found a little helper class called SqlMethods, which can be very usefull in such scenarios. SqlMethods has a method called Like, that can be used in a Linq to SQL query:

var query = from c in ctx.Customers

where SqlMethods.Like(c.City, "L_n%")

select c;

This method gets the string expression to check (the customer's city in this example) and the patterns to test against which is provided in the same way you'd write a LIKE clause in SQL.

Using the above query generated the required SQL statement:

SELECT CustomerID, CompanyName, ...
FROM dbo.Customers
WHERE City LIKE [L_n%]

Enjoy!