select t1.*, A = (select count(*) from table2 as t2 where t2.KEY = t1.KEY) from table1 as t1
var s = table1.Select((n1, n2) => new { field1 = n1.field1, field2 = n1.field2, count = table2.Where(n3 => n3.Key == n1.Key).Count() });
