LINQ在哪里加入条款 | LINQ在哪里加入条款

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问题描述

我在使用join/where子句和一个相当简单的sql select语句苦苦挣扎.

I'm struggling with a join/where clause with what is a rather simple sql select statement.

我正在尝试从tb1检索产品信息列表，其中where条件位于tbl2中，但这必须由三个不同的列连接起来.

I am trying to retrieve a list of product information from tb1 with the where condition behind situated in tbl2 but this must be joined by three different columns.

因此SQL看起来类似于:

so the SQL would look something along the lines of:

SELECT     tb1.*
FROM         tb2 INNER JOIN
                      tb1 ON tb2.Col1 = tb1. Col1 AND tb2.Col2 = tb1. Col2 AND
                      tb2.Col3 = tb1.Col3
WHERE     (tb2.Col1 = col1) AND (tb2.Col2 = col2) AND (tb2.Col4 = string)

ColX是主要的where子句，带有要作为参数传递的字符串；所有其他列都在上下文中.

ColX is the main where clause with the string to be passed in as parameter; all other columns are within the contexts.

如何使用where子句实现多个联接?

How do you implement multiple joins with a where clause?

朝正确的方向推，非常感谢.

And shoves in the right direction, muchly appreciated.

推荐答案

要在LINQ中的多个字段上进行联接，您必须创建一个包含要比较的列的新匿名类型，然后在该联接中使用该匿名类型:

To join on multiple field in LINQ, you have to create a new anonymous type containing the columns you want to compare and then use that anonymous type in the join:

var results = from t1 in context.tb1
              join t2 in context.tb2
              on new { t1.Col1, t1.Col2, t1.Col3 } equals
                  new { t2.Col1, t2.Col2, t2.Col3 }
              where t2.Col1 == col1 && t2.Col2 == col2 && t2.Col4 == someString
              select t1;

这是等效的Lambda语法:

And here is the equivalent Lambda Syntax:

var results = context.tb1.Join(
                  context.tb2,
                  t1 => new { t1.Col1, t1.Col2, t1.Col3 },
                  t2 => new { t2.Col1, t2.Col2, t2.Col3 },
                  (t1, t2) => new { t1, t2 })
              .Where(o => o.t2.Col1 == col1
                  && o.t2.Col2 == col2
                  && o.t2.Col4 == someString)
              .Select(o => o.t1);

如您所见，对于联接，查询语法通常会产生易于阅读的语句.

As you can see, in the case of joins, query syntax usually produces an easier to read statement.

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