使用Linq实现SQL的全连接

时间:2023-01-20 20:56:04

在*上看到关于这个问题的讨论,打算记录下来。

关于全连接(Full Outer Join),Linq并不支持,所以就需要使用其它方法来实现,就像不支持全连接的数据库一样,先使用内连接找出公共的部分,然后分别找出左连接和右连接的部分,把这三部分的结果UNION一下,即可得到全连接的效果。

看问题:

ID  FirstName
--  ---------
 1  John
 2  Sue

ID  LastName
--  --------
 1  Doe
 3  Smith
显示下面的结果:
ID  FirstName  LastName
--  ---------  --------
 1  John       Doe
 2  Sue
 3             Smith

 

实现方法一:

先找出左连接的结果,再找出右连接的结果,然后把这两个结果UNION即可,UNION时会自动把重复的数据过滤掉。

            var firstNames = new[]
            {
                new { ID = 1, Name = "John" },
                new { ID = 2, Name = "Sue" },
            };
            var lastNames = new[]
            {
                new { ID = 1, Name = "Doe" },
                new { ID = 3, Name = "Smith" },
            };
            var leftOuterJoin = from first in firstNames
                                join last in lastNames
                                on first.ID equals last.ID
                                into temp
                                from last in temp.DefaultIfEmpty(new { first.ID, Name = default(string) })
                                select new
                                {
                                    first.ID,
                                    FirstName = first.Name,
                                    LastName = last.Name,
                                };
            var rightOuterJoin = from last in lastNames
                                 join first in firstNames
                                 on last.ID equals first.ID
                                 into temp
                                 from first in temp.DefaultIfEmpty(new { last.ID, Name = default(string) })
                                 select new
                                 {
                                     last.ID,
                                     FirstName = first.Name,
                                     LastName = last.Name,
                                 };
            var fullOuterJoin = leftOuterJoin.Union(rightOuterJoin);

方法二:

得到左连接的数据,再从右连接的数据中把左连接的数据排除掉,两者的数据Concat一下即可

            var firstNames = new[]
            {
                new { ID = 1, Name = "John" },
                new { ID = 2, Name = "Sue" },
            };
            var lastNames = new[]
            {
                new { ID = 1, Name = "Doe" },
                new { ID = 3, Name = "Smith" },
            };
            var leftData = (from first in firstNames
                            join last in lastNames on first.ID equals last.ID into temp
                            from last in temp.DefaultIfEmpty(new { first.ID, Name = default(string) })
                            select new
                            {
                                first.ID,
                                FirstName = first.Name,
                                LastName = last.Name,
                            });

            var rightRemainingData = (from r in lastNames
                                      where !(from a in leftData select a.ID).Contains(r.ID)
                                      select new
                                      {
                                          r.ID,
                                          FirstName = default(string),
                                          LastName = r.Name
                                      });

            var fullOuterjoinData = leftData.Concat(rightRemainingData);

以上两种方法,其实是同一种方式来实现全连接的,只是第二次拿出的数据有所不同。

以上,希望对大家有所帮助。

相关文章