LINQ:一个“在哪里"子句与多条链接的"where子句" [英] LINQ: One "where" clause versus multiple chained "where clauses"

问题描述

我想知道下一件事:

我可以使用LINQ To Entities查询数据库，如下所示:

I can query my database using LINQ To Entities, like this:

GetAll().Where(
      x => x.SomeProperty == 'Yes'
      && x.SomeOtherProperty == 'No')
.ToList();

虽然我看到我的一些同事更改了这两个WHERE子句，例如:

While I see some of my colleagues change these two WHERE-clauses, like this:

GetAll().Where(x => x.SomeProperty == 'Yes')
      .Where(x.SomeOtherProperty == 'No')
.ToList();

两个查询都应该产生相同的输出，但是我想知道两者之一是否有优点/缺点.例如，一种方法会产生较慢的数据库查询，还是会生成相同的sql查询?

Both queries should produce the same output, but I was wondering if one of the two has any advantages/disadvantages. For instance, does one method produce a slower database query, or would they generate the same sql-query?

推荐答案

我使用

I set up a test project using the model and context described in this article and logged the SQL for two different queries, following the pattern in your question. The queries I made were:

db.Blogs
    .Where(b => b.BlogId == 0)
    .Where(b => b.Name == "Foo");

和

db.Blogs
    .Where(b => b.BlogId == 0 && b.Name == "Foo");

两个查询生成的SQL是相同的:

The generated SQL for both queries is identical:

SELECT 
    [Extent1].[BlogId] AS [BlogId], 
    [Extent1].[Name] AS [Name]
    FROM [dbo].[Blogs] AS [Extent1]
    WHERE (0 = [Extent1].[BlogId]) AND (N'Foo' = [Extent1].[Name])

因此(至少对于这样的简单情况)，似乎没有一种或另一种明显的性能差异.我猜您可能会争辩说，如果使用多重 Where 方法，则表达式访问者需要更长的时间来检查您的树，但是从长远来看，这可以忽略不计.

So it seems (at least for simple cases like this), there is no significant performance difference one way or the other. I guess you could argue it takes the expression visitor a bit longer to examine your tree if you use the multiple Where approach, but this is negligible in the long run.

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