Linq-to-Sql LEFT OUTER JOIN 在 2 个独立的 INNER JOINRS 之间

Question

Trying to generate the following SQL from Linq-To-Sql

SELECT 
parenttableA.Name AS parentA,
childtableA.Name AS childA,
parenttableB.Name AS parentB,
childtableB.Name AS childB

FROM parenttableA
INNER JOIN childtableA ON childtableA.parentid = parenttableA.id

LEFT OUTER JOIN

(
parenttableB
INNER JOIN childtableB ON childtableB.parentid = parenttableB.id
)

ON parenttableB.townid = parenttableA.townid

WHERE parenttableA.townid = 123

This SQL should return something like:

parentA    childA    parentB    childB
=======    ======    =======    ======
John       Dave      Paul       Mark
Jim        John      (null)     (null)

So, in other words:

How can I have 2 separate inner joins linked together with a LEFT OUTER JOIN so that I still get "A" records from town 123, even if no "B" records exist for that town?

Answer 1

This should work for IQueryable for enumerable you should add null checks

var query = from ptA in parenttableA
            join ctA in childtableA on ptA.Id equals ctA.Id
            join ptBctB in (from ptB in parenttableB
                            join ctB in childtableB on ptB.Id equals ctB.Id
                            select new { ptB, ctB }) on ptA.TownId equals ptBctB.ptB.TownId into ptBGroup
            from ptBctBLeft in ptBGroup.DefaultIfEmpty()
            where ptA.TownId == 123
            select new
                   {
                       parentA = ptA.Name,
                       childA = ctA.Name,
                       parentB = ptBctBLeft.ptB.Name,
                       childB = ptBctBLeft.ctB.Name,
                   };

Also I think you can do this with just mapping configuration and have this as a property in your POCO but you need to post the EF config for that.