LINQ - 查询查询结果（一些复杂的）

Question

Need some help in writing LINQ from the following SQL. The main problem is double grouping. I'm stacked in the second grouping

group by s.[e-year], s.[e-month]

Don't know how to implement.

Thanks a lot.

select s.[e-year], s.[e-month], count(s.projectid) 'projects entranced',
          ---------------------------------------------
          (select count(subquery.CustomerTypeID) from
              (select count(ap.ProjectID) as 'count', c.CustomerTypeID FROM Logging_ProjectsEntrances] pe
              inner join users u on pe.userid = u.userid
              inner join Companies c on u.CompanyId = c.CompanyID
              inner join AssignedProjects up on pe.ProjectID = up.ProjectID
              inner join Projects p on up.ProjectID = p.ProjectID
              where ap.ProductID = 1 and year(pe.EntranceDate) = s.[e-year] and MONTH(pe.entrancedate) = s.[e-month] and c.CustomerTypeID = 2
              group by ap.ProjectID, c.CustomerTypeID) subquery
          group by subquery.CustomerTypeID
          )
          --------------------------------------------
  from
  (
      select YEAR(pe.EntranceDate) as 'e-year', MONTH(pe.EntranceDate) as 'e-month', up.ProjectID as 'projectid' 
      FROM Logging_ProjectsEntrances pe
      inner join AssignedProjects ap on pe.ProjectID = ap.ProjectID
      inner join Projects p on ap.ProjectID = p.ProjectID
      where ap.ProductID = 1
      group by year(pe.EntranceDate), month(pe.EntranceDate), ap.ProjectID
  ) as s
  group by s.[e-year], s.[e-month]
  order by s.[e-year] desc , s.[e-month] desc

Answer 1

For translating SQL to LINQ query comprehension:

1. Translate FROM subselects as separately declared variables.
2. Translate each clause in LINQ clause order, translating monadic operators ( DISTINCT , TOP , etc) into functions applied to the whole LINQ query.
3. Use table aliases as range variables. Use column aliases as anonymous type field names.
4. Use anonymous types ( new { ... } ) for multiple columns.
5. Left Join is simulated by using a into join_variable and doing another from from the join variable followed by .DefaultIfEmpty() .
6. Replace COALESCE with the conditional operator and a null test.
7. Translate IN to .Contains() and NOT IN to ! ... Contains()
8. SELECT * must be replaced with select range_variable or for joins, an anonymous object containing all the range variables.
9. SELECT fields must be replaced with select new { ... } creating an anonymous object with all the desired fields or expressions.
10. Proper FULL OUTER JOIN must be handled with an extension method.

Note: Your SQL query is using a SQL trick ( SELECT x ... GROUP BY x ) to perform the equivalent of a DISTINCT , which should be used as it expresses the intent more clearly.

So, for your SQL query:

var subq = (from pe in projectsEntrances
            join ap in assignedProjects on pe.ProjectID equals ap.ProjectID
            join p in projects on ap.ProjectID equals p.ProjectID
            where ap.ProductID == 1
            select new { e_year = pe.EntranceDate.Year, e_month = pe.EntranceDate.Month, ap.ProjectID }).Distinct();

var ans = from s in subq
          group s by new { s.e_year, s.e_month } into sg
          orderby sg.Key.e_year descending, sg.Key.e_month descending
          select new { sg.Key.e_year, sg.Key.e_month, ProjectsEntranced = sg.Count() };