Question

I have 2 tables A{int id,int grp}, B{int aid,int cat}.

Table B contains list of categories that record of table A belongs to, so B.aid is Foreign Key that references A.id.

A.id is unique primary key of table A.

B.cat contains category number from 1 to 5, A.grp contains numbers from 1 to 1000.

Table A has 3 million of records, table B – about 5 million.

For each group A.grp I need to calculate % of records in A that contain B.cat out of number of records within group A.grp.

So if A:[{1,1},{2,1},{3,2}], B:[{1,3},{1,4},{2,3},{3,4}] then result of the query should be the following 3 column table:
R{int grp,int cat,double percent}:[{1,3,100},{1,4,50},{2,4,100}]

How can I do it with one single query in Linq ?

It is desired that A to appear only once in that query because I want to be able to replace A with A.Where(e=>some complicated expression) without duplicating it many times in that single query.

Tables A and B are imported into Linq to Entities with foreign keys so that it’s possible to reference from a in A from b in a.B select b.cat or from b in B select b.A.grp

Answer 1

You can combine your queries like this

var query = from g in 
              (from a in db.A
               group a by new
               {
                 grp = a.grp
               }
              )
            join c in  
              (from a in db.A
               from b in a.B
               group b by new
               {
                 a.grp,
                 b.cat
               }
              )            
            on g.Key.grp equals c.Key.grp
            select new
            {
              g.Key.grp,
              c.Key.cat,
              percent = c.Count() * 100 / g.Count()
            };