Question

I am just wondering about how we can determine whether to use join or not in linq to sql.

Eg. let say if we have two tables like this

Table 1   Customer
          id
          name

Table 2   addresstype
          id
          address1
          customerid

and

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

or

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

Is both way are the same. Is there any difference in performance?

Also the second method, will it come up with an error if there isn’t any matching?

Answer 1

Basically, these two LINQ queries are equivalent to the following SQL queries:

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

and

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

The difference between these two queries is mostly semantic, since the database will do the same thing in both cases and return a same result set for both queries.

I would prefer the join syntax in both SQL and LINQ since it defines an explicit relationship between the two tables/entities, that is only implied in the join-less version.