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

DefaultIfEmpty()需要LEFT JOIN，然后您可以检查加入的值是否为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

更容易就是这样写：

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

Answer 3

更快的方式

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