C# LINQ延迟执行并在左外部联接中返回null

我有两个问题，请看以下代码：

var table1 = from a in test1
             join b in test2 on a.column1 equals b.column1 into jo
             from jb in jo.DefaultIfEmpty()
             select new
             {
                col1 = jb == null ? <question 1> : jb.column1
             }

var table2 = from a in test3
             join b in test4 on a.column1 equals b.column1 into jo
             from jb in jo.DefaultIfEmpty()
             select new
             {
                col1 = jb == null ? <question 1> : jb.column1
             }

var table3 = from x in table1
             join y in table2 on x.col1 equals y.col1 into jo
             from jb in jo.DefaultIfEmpty()
             {
                  col1 = jb == null ? <question 1> : jb.col1
             }

var table4 = from z in table1.Concat(table2)
             select new TheTables()
             {
                 column1 = x.col1
             }

---

第二个问题：根据我上面给出的代码，当我执行“table4.List（）”时，它会以组合语句的形式查询数据库，还是会查询table1，然后查询table2，然后将其作为table3加入内存？

我想你的意思是从jb in jo.DefaultIfEmpty（）哈哈哈，很抱歉我错过了这一点，谢谢你的提示。：）

The argument to DbIsNullExpression must refer to a primitive, enumeration or reference type.