您如何使用 linq 离开加入“非空”？答案

【问题标题】：How do you Left join 'Is Not Null' using linq?您如何使用 linq 离开加入“非空”？
【发布时间】：2021-02-09 21:27:07
【问题描述】：

我在让我的 LINQ 语句正常工作时遇到了一些问题。我要加入一个表 secondTable，其中一个列可以为空，但我只需要该列不为空的记录。我不确定如何将以下内容放入 LINQ 表达式中

LEFT JOIN secondTable b ON a.ID = b.oneTableID AND b.name IS NOT NULL

到目前为止，我的 LINQ 是：

var list = await (from one in dbRepository.oneTable
                  join two in dbRepository.secondTable
                  on new { name = one.name, phone = one.phone, height = { is not null} } equals new 
                  { name = two.name, phone = two.phone, height = two.height 
                  into temp
                  from two in temp.DefaultIfEmpty()
                  select new.....

有什么想法吗？

编辑 1：我找到了解决方案。

 var list = await (from one in dbRepository.oneTable
                  join two in dbRepository.secondTable
                  on new { name = one.name, phone = one.phone, height = false } equals new 
                  { name = two.name, phone = two.phone, height = string.IsNullOrEmpty(two.height)}
                  into temp
                  from two in temp.DefaultIfEmpty()
                  select new.....

【问题讨论】：

标签： c# sql-server linq

【解决方案1】：

您必须使用SelectMany 来创建 LEFT JOIN：

var query = 
    from one in dbRepository.oneTable
    from two in dbRepository.secondTable
        .Where(two => two.name = one.name && two.phone == one.phone 
            && two.height != null)
        .DefaultIfEmpty()
    select new.....

【讨论】：

【解决方案2】：

试试这个：

var list = await (from one in dbRepository.oneTable
                  join two in dbRepository.secondTable
                  on new { name = one.name, phone = one.phone} 
                  equals new 
                  { name = two.name, phone = two.phone}
                  into temp
                  from two in temp.DefaultIfEmpty()
                  where one.height == null || one.height = two.height 
                  select new.....

【讨论】：