如何使用LINQ实现左排除JOIN？

Question

如何使用LINQ实现左排除JOIN？

在SQL中:

SELECT <select_list> 
FROM Table_A A
LEFT JOIN Table_B B
ON A.Key = B.Key
WHERE B.Key IS NULL

Answer 1

你需要DefaultIfEmpty()的LEFT JOIN,那么你可以检查是否加盟值是null:

var result = from a in Table_A
             join b in Table_B on a.Key equals b.Key into j
             from b in j.DefaultIfEmpty()
             where b == null
             select new { ... };

Answer 2

更容易写这样:

var result = from a in Table_A
             where !Table_B.Any(b => b.Key == a.key)
             select new { ... };