Question

我需要写一个Linq查询，其条件如下：

当数据存在时，联接将为INNER JOIN
当数据不存在时，联接将为LEFT JOIN。

bool ifexists =  db.Users.Any(u => u.id== 1);
IQueryable<modelname> query;
if(ifexists == true)
{
    query=my inner join linq;
    query = from s in query select s;
    return query.ToList();
}
else 
{
    query=my left join linq;
    query = from s in query select s;
    return query.ToList();
}

现在我怎么能写这个Linq？有没有办法在单Linq语句中编写这个linq - 我不想写两次linq？

Answer 1

在对数据库进行评估之前，您可以有条件地在IQueryable上进行撰写 - 这将允许您至少部分地干掉分支的不同分支上的常见操作：

var query = db.SomeTable
    .AsQueryable() // Not needed if `db.SomeTable` is already a Queryable
    .Where(x => ** common predicates on x ** ));
    // NB : Do not materialize here!

if(exists) // Don't need == true
{
     query = query
      .Where(x => ** branch 1 predicates **));
}
else
{
     query = query
      .Where(x => ** branch 2 predicates **);
}
return query
  .Select(q => ** common fields to project **)
  .OrderBy(q => ** common ordering **)
  .ToList(); // Materialize right at the end

Answer 2

你想要那样吗？

bool ifexists =  db.Users.Any(u => u.id== 1);

var query=ifexists==true ? (my inner join linq) : (my left join linq);

return query.ToList();

Answer 3

   First create object of of database Context named **Objcontext** 
    now hold the variable in var the query would be like following 




 var Result = from a in Objcontext.tbl1  where !Objcontext.tbl2.Any(m=>(m.sid==a.aid)&&(m.mid==80))
    select a; 



 If data is exists in the databse it will cheack using !Objcontext 
    I hope it will helpfull

如何在不重复的情况下编写条件（IF DATA EXISTS）LINQ语句？

3 个答案: