转换SQL左连接到LINQ Lambda表达式

Question

我有这样的SQL查询

select at.description, 
     a.Address1, 
     a.Address2, 
     a.City 
    from address_types at 
    left join Address a 
    on a.addresstype = at.addresstype 
    and a.addressid=24 

想它写在LINQ的Lambda表达式，任何线索？

谢谢！

Answer 1

我宁愿把它简化以下列方式

(from at in address_types 
join a in address 
on at.addresstype equals a.addresstype into tt 
from ar in tt.DefaultIfEmpty() 
where (ar != null && ar.addressid == 24) 
select new 
{ 
    at.Description, 
    ar.Address1, 
    ar.Address2, 
    ar.City 
    }) 

为什么我们不需要这种检查的理由“（JA == NULL） “是因为我们已经在这一行限制了我们的设置”（ar！= null & & ar.addressid == 24）“。

同时这个查询的工作原理，我的POV是，在SQL查询中，LEFT JOIN也是冗余的。 http://msdn.microsoft：你会不会因为你的条件“a.addressid = 24”，所以SQL可以改写到INNER JOIN和LINQ查询以及对

(from at in address_types 
    join a in address 
    on at.addresstype equals a.addresstype 
    where at.addressid == 24 
    select new 
    { 
     at.Description, 
     a.Address1, 
     a.Address2, 
     a.City 
     }) 

Answer 2

你可以尝试这样的事情：

var resultset = (from at in addresstypes 
        join a in addresses 
        on at.AddressType equals a.AddressType into joinaddress 
        from ja in joinaddress.DefaultIfEmpty() 
        where (ja != null && ja.AddressID == 24) 
        select new 
        { 
         AddressType = at.AddressType, 
         AddressID = ja == null ? 0 : ja.AddressID, 
         Address1 = ja== null ? string.Empty : ja.Address1 
         });