Question

当1个或多个用户保存相同产品时，左联接返回重复的产品。

我解决了SQL查询中的问题。这是一个查询：

select p.ProductID,  
       (case when c.UserID = 3 then 'true' else 'false' end) as flag
    from product as p  
    left join SavedItem as c on product.ProductID = c.ProductID and
      c.UserID = 3

无法弄清楚如何在实体框架中进行操作。

left join SavedItem as c on product.ProductID = c.ProductID and
 c.UserID = 3

左联接可以这样解决：

join c in SavedItem on p.ProductID equals c.ProductID into lj
    from c in lj.DefaultIfEmpty()

c.UserID = 3放在哪里？

Answer 1

尝试一下：

var query=  from product in context.Products
            from SavedItem in context.SavedItems.Where(c=> c.ProductID = product.ProductID && c.UserID == 3).DefaultIfEmpty()
            select new {
                        ProductID=product.ProductID,
                        Flag=(SavedItem==null || SavedItem.UserID != 3) ? false : true
            };

Answer 2

在这里！我找到了解决方法。

from product in context.Products
      join c in context.SavedItems 
      on new { p1 = (int?)product.ProductID , p2 = (int?)cat.UserID } 
      equals new { p1 = c.ProductID ,p2 = c.UserID} into lj
      from c in lj.DefaultIfEmpty()
                   select new{
                        ...
                       flag = (c.UserID == cat.UserID ? "true" : "false"),
                         ...
                             }

左联接仅返回用户ID匹配的值

2 个答案: