Sort by New Guid for random order

问题

In order to get results in a random order from a Sql query I generally sort by new Guids. I have done this before with Entity-Framework, however for some reason its not working now.

For example (using the adventureworks2008r2 database) i run the following query in LinqPad:

(from t in Employees
 orderby Guid.NewGuid()
 select new {t.Person.FirstName,t.Person.LastName,t.JobTitle})

This generates the following SQL:

SELECT [t1].[FirstName], [t1].[LastName], [t0].[JobTitle]
FROM [HumanResources].[Employee] AS [t0]
INNER JOIN [Person].[Person] AS [t1] ON
   [t1].[BusinessEntityID] = [t0].[BusinessEntityID]

So what happend to my orderby query?

I took this one step further with the following query to find that Guid.NewGuid() is only being called once.

(from r in (from t in Employees
 select new {t.Person.FirstName,t.Person.LastName,t.JobTitle,
    g = Guid.NewGuid()})
 orderby r.g
 select r)

This generated the following SQL query

-- Region Parameters
DECLARE @p0 UniqueIdentifier = '68ad5016-19ca-4e31-85c3-1d45618ea8c9'
-- EndRegion
SELECT [t2].[FirstName], [t2].[LastName], [t2].[JobTitle]
FROM (
    SELECT [t1].[FirstName], [t1].[LastName], [t0].[JobTitle], @p0 AS [value]
    FROM [HumanResources].[Employee] AS [t0]
    INNER JOIN [Person].[Person] AS [t1] ON
  [t1].[BusinessEntityID] = [t0].[BusinessEntityID]
    ) AS [t2]
ORDER BY [t2].[value]

Any idea whats going on?

回答1:

I believe the problem is caused by way LinqPad is creating the DBContext (or whatever it does internally) when you query a database directly (as opposed to creating your own EF connection). If I run this:

using (var context = new MyContext()) {

    var query =
    from x in context.MyTable
    select new {
        x.ID,
        g = Guid.NewGuid()
    };

}

I get the following SQL

SELECT 
[Extent1].[ID] AS [ID], 
NEWID() AS [C1]
FROM [dbo].[MyTable] AS [Extent1]

Which results in a unique guid for each row. You can alter the Linq to orderby Guid.NewGuid() and you'll get the random sorting that you want.

var query =
    from x in context.MyTable
    orderby Guid.NewGuid()
    select new {
        x.ID
    };

来源：https://stackoverflow.com/questions/20953067/sort-by-new-guid-for-random-order

标签

sql

linq

entity-framework

linqpad