How to determine whether to use join in linq to sql?
Table 1   Customer
          id
          name

Table 2   addresstype
          id
          address1
          customerid

var address = from cu in Customer
              from ad in addresstype
              where cu.id == ad.customerid
              select ad;

var address = from cu in Customer
              join ad in addresstype on cu.id equals ad.customerid
              select de;

select ad.*
 from Customer cu, AddressType ad
 where cu.ID == ad.CustomerID -- I assume this was meant by the OP

select ad.*
from Customer cu
  inner join AddressType ad on cu.id = ad.CustomerID;

new {X,Y} equals new {X',Y'}

var query = from cu in Customer
              from ad in cu.Addresses
              select ad;