Random row from Linq to Sql

Question

What is the best (and fastest) way to retrieve a random row using Linq to SQL when I have a condition, e.g. some field must be true?

Answer 1

i use this method for take random news and its work fine ;)

    public string LoadRandomNews(int maxNews)
    {
        string temp = "";

        using (var db = new DataClassesDataContext())
        {
            var newsCount = (from p in db.Tbl_DynamicContents
                             where p.TimeFoPublish.Value.Date <= DateTime.Now
                             select p).Count();
            int i;
            if (newsCount < maxNews)
                i = newsCount;
            else i = maxNews;
            var r = new Random();
            var lastNumber = new List<int>();
            for (; i > 0; i--)
            {
                int currentNumber = r.Next(0, newsCount);
                if (!lastNumber.Contains(currentNumber))
                { lastNumber.Add(currentNumber); }
                else
                {
                    while (true)
                    {
                        currentNumber = r.Next(0, newsCount);
                        if (!lastNumber.Contains(currentNumber))
                        {
                            lastNumber.Add(currentNumber);
                            break;
                        }
                    }
                }
                if (currentNumber == newsCount)
                    currentNumber--;
                var news = (from p in db.Tbl_DynamicContents
                            orderby p.ID descending
                            where p.TimeFoPublish.Value.Date <= DateTime.Now
                            select p).Skip(currentNumber).Take(1).Single();
                temp +=
                    string.Format("<div class=\"divRandomNews\"><img src=\"files/1364193007_news.png\" class=\"randomNewsImg\" />" +
                                  "<a class=\"randomNews\" href=\"News.aspx?id={0}\" target=\"_blank\">{1}</a></div>",
                                  news.ID, news.Title);
            }
        }
        return temp;
    }

Answer 2

One way to achieve efficiently is to add a column to your data Shuffle that is populated with a random int (as each record is created).

The partial query to access the table in random order is ...

Random random = new Random();
int seed = random.Next();
result = result.OrderBy(s => (~(s.Shuffle & seed)) & (s.Shuffle | seed)); // ^ seed);

This does an XOR operation in the database and orders by the results of that XOR.

Advantages:-

1. Efficient: SQL handles the ordering, no need to fetch the whole table
2. Repeatable: (good for testing) - can use the same random seed to generate the same random order

This is the approach used by my home automation system to randomize playlists. It picks a new seed each day giving a consistent order during the day (allowing easy pause / resume capabilities) but a fresh look at each playlist each new day.

Answer 3

To add to Marc Gravell's solution. If you are not working with the datacontext class itself (because you proxy it somehow e.g. to fake the datacontext for testing purposes), you cannot use the defined UDF directly: it will not be compiled to SQL because you're not using it in a subclass or partial class of your real data context class.

A workaround for this problem is to create a Randomize function in your proxy, feeding it with the query you want to be randomized:

public class DataContextProxy : IDataContext
{
    private readonly DataContext _context;

    public DataContextProxy(DataContext context)
    {
        _context = context;
    }

    // Snipped irrelevant code

    public IOrderedQueryable<T> Randomize<T>(IQueryable<T> query)
    {
        return query.OrderBy(x => _context.Random());
    }
}

Here is how you'd use it in your code:

var query = _dc.Repository<SomeEntity>();
query = _dc.Randomize(query);

To be complete, this is how to implement this in the FAKE datacontext (which uses in memory entities):

public IOrderedQueryable<T> Randomize<T>(IQueryable<T> query)
{
    return query.OrderBy(x => Guid.NewGuid());
}

Answer 4

Came here wondering how to get a few random pages from a small number of them, so each user gets some different random 3 pages.

This is my final solution, working querying with LINQ against a list of pages in Sharepoint 2010. It's in Visual Basic, sorry :p

Dim Aleatorio As New Random()

Dim Paginas = From a As SPListItem In Sitio.RootWeb.Lists("Páginas") Order By Aleatorio.Next Take 3

Probably should get some profiling before querying a great number of results, but it's perfect for my purpose

Answer 5

var cust = (from c in ctx.CUSTOMERs.ToList() select c).OrderBy(x => x.Guid.NewGuid()).Taket(2);

Select random 2 row

Answer 6

You can do this at the database, by using a fake UDF; in a partial class, add a method to the data context:

partial class MyDataContext {
     [Function(Name="NEWID", IsComposable=true)] 
     public Guid Random() 
     { // to prove not used by our C# code... 
         throw new NotImplementedException(); 
     }
}

Then just order by ctx.Random(); this will do a random ordering at the SQL-Server courtesy of NEWID(). i.e.

var cust = (from row in ctx.Customers
           where row.IsActive // your filter
           orderby ctx.Random()
           select row).FirstOrDefault();

Note that this is only suitable for small-to-mid-size tables; for huge tables, it will have a performance impact at the server, and it will be more efficient to find the number of rows (Count), then pick one at random (Skip/First).

---

for count approach:

var qry = from row in ctx.Customers
          where row.IsActive
          select row;

int count = qry.Count(); // 1st round-trip
int index = new Random().Next(count);

Customer cust = qry.Skip(index).FirstOrDefault(); // 2nd round-trip