在 Linq 中使用 IsNull 或选择 COALESCE ..?-数据库问题

Using IsNull or select COALESCE in Linq..?(在 Linq 中使用 IsNull 或选择 COALESCE ..?)

本文介绍了在 Linq 中使用 IsNull 或选择 COALESCE ..?的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

可能的重复:
LINQ中SQL ISNULL的等价物?

我有 3 张这样的桌子:

I have 3 tables like this :

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，以便结果像这样返回:
I want to make a query so the result to return like this :
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
then others provided this solution queries 
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 Management 中都可以正常工作，但在 Linq 中使用:
both works fine in SQL Management , but in Linq with the:
ObjectQuery<string> results1 = context.CreateQuery<string>(query1, parameters) 

我已经尝试将它自己转换为:
I've tried to convert it my self to :
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
  };

但它返回空
有什么想法吗?
推荐答案

看来你在 tmp1 附近有错字，你应该做 tree1.DefaultIfEmpty() >

It seems you have a typo near tmp1, you should do 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
      };

这篇关于在 Linq 中使用 IsNull 或选择 COALESCE ..?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持编程学习网！