如何在 Linq (C# MVC) 中用 case 编写左连接

Question

如何在 Linq 中编写以下查询：

SELECT s.displayname AS Skill, 
       CASE 
         WHEN us.skillid IS NULL THEN '0' 
         ELSE '1' 
       END           AS HasSkill 
FROM   skills s 
       LEFT JOIN userskills us 
              ON s.id = us.skillid 

我已经研究了几个小时，我真的无法理解这些查询的结构。对于普通 SQL 查询似乎对我所做的事情来说，它们似乎势不可挡且过于复杂。

var skills = _context.Skills.ToList();
var userSkills = _context.UserSkills.ToList();

var result = skills.GroupJoin(userSkill, skill => skill.ID, skill => userSkill , (userSkill, skill) => new {
                Key = userSkill,
                Skills = skill
            });

Answer 1

您可以尝试使用linq 和DefaultIfEmpty() 制作LEFT JOIN

然后将select与三元运算符一起使用，使CASE WHEN

from s in skills
join u in userSkills 
on new s.id equals u.skillid 
into temp
from u in temp.DefaultIfEmpty()
select new { 
    Skill=s.displayname,
    HasSkill= us.skillid == NULL ? "0" : "1"
}