Linq - get top 2 records through criteria - sql

Question

i have data in following format:
colA colB
2010 10
2010 20
2010 30
1999 99
I need to obtain output as follows using Linq/Lambda expression:
colA colB
2010 10
1999 99
colB could be any non-99 value but the top 2 should always list 99 and any 1 instance of non-99 value.
I am after the values in colA.
TIA

Answer 1

Are you going for something like this?
var value1 = data.FirstOrDefault(x => x.colB != 99).Select(x => x.colA);
var value2 = data.FirstOrDefault(x => x.colB == 99).Select(x => x.colA);
Which could be combined into an IEnumerable using a utility method such as the following extension:
public static void CreateEnumerable<TResult>(this TResult obj)
{
yield return obj;
}
In a manner like so:
var results = value1.CreateEnumerable().Concat(value2.CreateEnumerable());

Answer 2

You'll want to learn how to group using GroupBy(); go to 101 LINQ Samples: Grouping Operators.
public class Record
{
public int colA; { get; }
public int colB; { get; }
}
...
// this extension method will clean up our searching within the group clause
public static Record Select99OrFirst(this IEnumerable<Record> source)
{
var item = source.FirstOrDefault(r => r.colB == 99);
return (null != item) ? item : source.First();
}
...
IEnumerable<Record> data;
var query = data.GroupBy(r => r.colA)
.Select(g => new { Year = g.Key, Value = g.Select99OrFirst().colB })
.OrderByDescending(x => x.Year);
Console.WriteLine("colA/tcolB");
foreach (var item in query)
Console.WriteLine(item.Year + "/t" + item.Value);
This will give you anonymous objects with a Year property and a Value property for colA and colB respectively. Each will represent the first record within a year group if it couldn't find the 99 record in the group first.