Convert SQL with multiple join into LINQ

问题

I would like to know how can i change the following sql statement into Linq or Lambda Extension in C#

SELECT m.mdes as AgeGroup,COUNT(DISTINCT(mbcd))as "No.of Member" FROM mageg m
LEFT JOIN  (select distinct(mbcd) ,mage
FROMtevtl
JOIN mvipm 
ON tevtl.mbcd = mvipm.mvip
WHERE datm >= '2014-04-01'
AND datm <= '2014-04-30'
)vip
ON m.tage >= vip.mage AND m.fage <= vip.mage
GROUP BY m.mdes

I manage to do the first half of the LINQ statement. Not sure If it is correct here is the first half. I do not know how to connect with the left join.

(from mem in mvipms
from log in tevtls
from grp in magegs
where mem.mage >=grp.fage && mem.mage <=grp.tage && mem.mvip.Equals(log.mbcd)
&& log.datm >= DateTime.Parse("2014-04-01") && log.datm <= DateTime.Parse("2014-04-30")
select new {mem.mvip,grp.mdes}).Distinct()

Pls advice. I am using the MSSQL 2008 and VS2010.

Thanks a million.

回答1:

I am no expert on LINQ, but since no-one else has answered, here goes!

Firstly you cannot (AFAIK) do a LINQ join on anything other than equals so the structure of your LEFT JOIN has to change. Partly for debugging purposes, but also for readability, I prefer to layout my LINQ in bite-size chunks, so what I would do in your case is something like this (assuming I have understood your data structure correctly):

    var vip = (from t in tevtl
               join v in mvipl
               on t.mbcd equals v.mvip
               where t.datm >= new DateTime(2014, 4, 1) && t.datm <= new DateTime(2014, 4, 30)
               select new { t.mbcd, v.mage }).Distinct();
    var mv = from m in magegl
             from v in vip
             where m.tage >= v.mage && m.fage <= v.mage
             select new
             {
                 m.mdes,
                 v.mbcd
             };
    var gmv = from m in mv
              group m by new { m.mdes } into grp
              select new
              {
                  mdes = grp.Key.mdes,
                  CountMBCD = grp.Count()
              };
    var lj = from m in magegl
             join v in gmv
             on m.mdes equals v.mdes into lhs
             from x in lhs.DefaultIfEmpty()
             select new
             {
                 AgeGroup = m.mdes,
                 CountMBCD = x != null ? x.CountMBCD : 0
             };

By way of explanation. mv is the equivalent structure for your left join in that it has the relevant where clause, but obviously it does not contain any nulls - it is equivalent to an INNER JOIN. gmv is a group on mv, so is still effectively an INNER JOIN. Then to pick up the missing mdes (if any) I re-join on magel with the added syntax DefaultIfEmpty() - this converts the join into the equivalent of a LEFT OUTER JOIN.

Without any sample data, I haven't been able to test this, but I hope it gives you enough to point you in the right direction!

来源：https://stackoverflow.com/questions/23646142/convert-sql-with-multiple-join-into-linq

标签

sql

linq

sql-to-linq-conversion