使用LINQ，C＃将两个条件左外部联接构造为一个查询

Question

Currently, I am executing two queries based upon whether w.Type is either 1 or 2. If w.Type is 1 we perform a join to the Issues table and if the Type is 2 we join to the TSubs table. I am trying to merge these queries into one.

var productIdOne = (from w in listAbandonedCarts
                    join i in Issues on w.ProductId equals i.Id
                    where w.Type == 1
                    select new { i.Title.Name }).ToList();

var productIdTwo = (from w in listAbandonedCarts
                    join ts in TSubs on w.ProductId equals ts.Id
                    where w.Type == 2
                    select new { ts.Title.Name }).ToList();

I am considering using two left outer joins based upon this SQL psuedo-code

SELECT*
FROM P_carts pc
LEFT OUTER tSubs ts on ts.id = pc.productid and pc.Type = 2
LEFT OUTER issues i on i.id = pc.productid and pc.Type = 1

So far i have some linq pseudo coded but i'm struggling to get the syntax correct for the two conditional joins

 var listProducts = (from w in listAbandonedCarts
                     join i in Issues on w.ProductId equals i.Id into iN && w.ProductId == 1
                     from i in iN.DefaultIfEmpty()
                     join  into ts in TSubs.....

The problem I'm struggling with is that this isn't a double left outer its two separate joins. My current error is that i cannot have w.ProductId after the equals because I'm out of scope of w at this point and can't figure out how to structure the linq statement. Any help would be much appreciated, thanks!

Answer 1

Give this a shot:

        var productIds = (
            from w in listAbandonedCarts
            from i in Issues.Where(issue => issue.Id == w.ProductId && w.Type == 1).DefaultIfEmpty()
            from t in TSubs.Where(tsub => tsub.Id == w.ProductId && w.Type == 2).DefaultIfEmpty()
            select new {Name = (w.Type == 1 ? i.Title.Name : t.Title.Name)})
            .ToList();

Answer 2

var listProducts = (from w in listAbandonedCarts
                 join i in Issues on w.ProductId equals i.Id && w.ProductId == 1 into iN 
                 from i in iN.DefaultIfEmpty()
                 join  into ts in TSubs on ts.id equals  pc.productid into tsBag
                from ts in tsBag.DefaultIfEmpty()
                select new
                {
                  // All columns you need 
                };  

Please try the above linq. This sample code is not compiled nor tested. Please use this for reference purpose only.