在 Linq 中使用 IsNull 或选择 COALESCE ..？ [复制]答案

【问题标题】：Using IsNull or select COALESCE in Linq..? [duplicate]在 Linq 中使用 IsNull 或选择 COALESCE ..？ [复制]
【发布时间】：2012-06-16 14:17:25
【问题描述】：

可能重复：
Equivalent of SQL ISNULL in LINQ?

我有 3 个这样的表：

评论表：

commentId pid    sid       text      vid
1          1     null    comment 1    1
2         null    1     comment 2    1
3          2     null    comment 3    1

学生桌

sid     firstname   lastname
1         john       adam
2         joan       adam

教授表：

pid    firstname   lastname
1       mark        abram
2       sean        hoak

我想进行查询，结果返回如下：

firstname    lastname
mark          abram
john          adam
sean          hoak

米

if (select query ==null) then (selec query 1) else select (query 2)

我尝试了以下方法：

if((select pid from comment==null)
then select student.firstname , student.lastname from student where sid in (select sid from comment where vid=1)

else
(select professor.firstname ,professor.lastname from professor where pid in (select pid from comment where vid=1)

但没有用

然后其他人提供了这个解决方案查询

IsNull 查询

SELECT ISNULL(P.firstname, s.firstname) AS Expr1,ISNULL(P.lastname, s.lastname) AS Expr2
FROM comment AS C 
    LEFT OUTER JOIN professor AS P ON P.ID = C.PID 
    LEFT OUTER JOIN student AS s ON s.ID = C.SID
WHERE (C.VID = 2)

合并：

  SELECT COALESCE(p.firstname, s.firstname), COALESCE(p.lastname, s.lastname)
  FROM comment c
  LEFT JOIN Professor p
  ON c.pid = p.id
 LEFT JOIN Student s
  ON c.sid = s.id
   WHERE c.vid = 2

在 SQL 管理中都可以正常工作，但在 Linq 中：

ObjectQuery<string> results1 = context.CreateQuery<string>(query1, parameters)

我尝试将它自己转换为：

var qry = from c in ctx.comment
  join p in ctx.professor
  on c.PID equals p.ID into tree
  join s in ctx.student
  on c.SID equals s.ID into tree1
  where c.VID == vID
  from tmp in tree.DefaultIfEmpty()
  from tmp1 in tree.DefaultIfEmpty()
  select new
  {
      Firstnmae = tmp.firstname ?? tmp1.firstname,
      LastName = tmp.lastname ?? tmp1.lastname
  };

但它返回 null

有什么想法吗？

【问题讨论】：

标签： sql sql-server linq tsql linq-to-sql

【解决方案1】：

您在tmp1 附近似乎有错字，您应该使用tree1.DefaultIfEmpty()

      var qry = from c in ctx.comment
      join p in ctx.professor
      on c.PID equals p.ID into tree
      join s in ctx.student
      on c.SID equals s.ID into tree1
      where c.VID == vID
      from tmp in tree.DefaultIfEmpty()
      from tmp1 in tree1.DefaultIfEmpty()
      select new
      {
          Firstnmae = tmp.firstname ?? tmp1.firstname,
          LastName = tmp.lastname ?? tmp1.lastname
      };

【讨论】：

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