Question

我试图在2个参数上做一个Left Out Join，但是失败了。

基本上，我需要的是

DECLARE @SomeValue
SET @SomeValue = 99

SELECT * FROM
[Table1] LEFT OUTER JOIN [Table2]
ON ([Table1].[Field1]=[Table2].[Field1] AND [Table2].[Field2]=@SomeValue)

如何使用LINQ执行此操作？

Answer 1

第二次......不确定我是否完全喜欢这种解决方案 - 但有一种方法是：

var someValue = 99;
var preQuery = from t2 in Table2
               where t2.Field2 == someValue
               select t2;

var query = from t1 in Table1
            join t2 in preQuery on t1.Field1 equals t2.Field1
            select new
            {
               T1 = t1,
               T2 = t2.FirstOrDefault()
            };

Answer 2

LINQ to SQL：

int someValue = 99; 
var query =
    from table1 from dc.Table1Items
    join table2 from dc.Table2Items
    on new { table1.Field1, Field2 = someValue } equals new { table2.Field1, table2.Field2 } into table2Items
    from table2 in table2Items.DefaultIfEmpty()
    select new
    {
       table1,
       table2,
    };

Answer 3

var someValue = 99;
var query = from t1 in Table1
            join t2 in Table2 
            on t1.Field1 equals t2.Field1 into tempTable
            from t3 in tempTable.Where(v => v.Field2 == someValue).DefaultIfEmpty()
            select new { t1, t3};

Answer 4

这是我最喜欢的左外连接语法：

from t1 in Table1
from t2 in Table2
    .Where(x => x.Field1 == t1.Field1 &&
                x.Field2 == someValue)
    .DefaultIfEmpty()
select new { t, t2 }

Answer 5

假设你想在Linq中使用它：

var someValue = 99;
var qry = from t1 in Table1
          join t2 in Table2 on t1.Field1 equals t2.Field1 into tempTable
          from t3 in tempTable.FirstOrDefault()
          where (t3==null || t3.Field2 == someValue)
          select new
          {
              T1 = t1,
              T2 = t3
          };

Answer 6

var query = from t1 in table1 join t2 in table2 equals t2.field2 = @somevalue into new table
        from t in table.DefaultIfEmpty() select new { t1, t2== null ? t1.field : t2.field };

LINQ SQL：2个参数的左外连接

6 个答案: