查询实体框架中的第n行

Question

I want to query a random row from a table per Entity Framework.

The best solution I can think of in the moment is getting the total count from the table, then making per C# a function to get a random number in the range of the count and then query this random row number.

My problem is that I can't figure out how to directly query a certain row number per Entity Framework. I want to prevent that I need to query the whole table and then select the row number from there.

Or do I have here a general misunderstanding and there is a much simpler way?

Answer 1

// DO NOT USE THIS FOR MORE THEN 100 ROWS
var randomRecord = foos.OrderBy( x=> SqlFunctions.Rand() ).FirstOrDefault();

But this method is less efficient then,

// USE THIS FOR MORE THEN 100 ROWS
var random = Math.Random(foos.Count());

var randomRecord = foos.OrderBy( x=> x.id ).Skip( random ).FirstOrDefault();

For database, querying count is much less overhead then actually performing SORT over RANDOM for thousands of records. As RANDOM is certainly not indexed, so it will take very long to sort. So avoid using first method, use 2nd method that is the best.

Answer 2

This will get you the (x+1)th row. If x is 0 based, then if x==0 then it will get the 1st row.

Take(n) will dictate the query to take n rows. Skip(n) will dictate the query to skip the first n rows.

Thanks to AkashKava it is clear that in EF you have to use OrderBy before applying Skip.

Since we don't care of a particular order then we can sorted like the following.

Table.OrderBy(x=>x.id).Skip(1).Take(1)

or as also pointed out by AkashKava you could use FirstOrDeafault()

Table.OrderBy(x=>x.id).Skip(1).FirstOrDeafault()