本文介绍了LINQ 加入 Where 子句的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!

问题描述

我在使用一个相当简单的 sql select 语句的 join/where 子句中苦苦挣扎.

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

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

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.

所以 SQL 看起来像这样:

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,字符串作为参数传入;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 子句实现多个连接?

How do you implement multiple joins with a where clause?

并且朝着正确的方向推进,非常感谢.

And shoves in the right direction, muchly appreciated.

推荐答案

要在 LINQ 中连接多个字段,您必须创建一个包含要比较的列的新匿名类型,然后在连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;

这里是等效的 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,在连接的情况下,查询语法通常会生成更易于阅读的语句.

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

这篇关于LINQ 加入 Where 子句的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持!

08-20 04:39
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