Question

我有一个表格，其中包含有关项目和版本的详细信息，如下所示：

+--------+-----------+------------+----------------------------+
| BookId | VersionId | InstanceId |         TitleText          |
+--------+-----------+------------+----------------------------+
|      1 |         1 |          1 | A great book!              |
|      1 |         2 |          2 | Great Book, v2!            |
|      2 |         1 |          3 | Cooking for Dummies        |
|      3 |         1 |          4 | Networks for Dummies       |
|      2 |         2 |          5 | Cooking for Smarter People |
+--------+-----------+------------+----------------------------+

我试图查询BookId最新（最高）VersionId的所有行。所以我想要的结果集如下：

+--------+-----------+------------+----------------------------+
| BookId | VersionId | InstanceId |         TitleText          |
+--------+-----------+------------+----------------------------+
|      1 |         2 |          2 | Great Book, v2!            |
|      3 |         1 |          4 | Networks for Dummies       |
|      2 |         2 |          5 | Cooking for Smarter People |
+--------+-----------+------------+----------------------------+

我正在运行的SQL是：

select t1.* from Books t1
left outer join Books t2
on (t1.BookId = t2.BookId and t1.VersionId < t2.VersionId)
where t2.BookId is null

但是经过一些谷歌搜索后，我无法弄清楚如何将其转换为linq查询（使用扩展方法sytax）。我看到的例子往往会加入两个不同的表格，我只是没有弄清楚如何将其塑造成我需要的。

发现这一点，它似乎接近我正在寻找的东西，但我如何为Foo和Bar做同样的桌子呢？

Foo.GroupJoin(
          Bar, 
          foo => foo.Foo_Id,
          bar => bar.Foo_Id,
          (x,y) => new { Foo = x, Bars = y })
    .SelectMany(
          x => x.Bars.DefaultIfEmpty(),
          (x,y) => new { Foo=x.Foo, Bar=y});

Answer 1

如何使用分组呢？

from x in myTable
group x by x.BookId into g
select g.OrderByDescending(y => y.VersionId).FirstOrDefault()

Answer 2

一种方法是将您的反连接转换为where-not-exists：

select t1.*
from Books t1
where not exists (
    select 1
    from Books t2
    where t2.BookId = t1.BookId and t2.VersionId > t1.VersionId
)

哪个应该转换为LINQ：

AllBooks.Where(t1 => 
    !AllBooks.Any(t2 => t1.BookId == t2.BookId && t2.VersionId > t1.VersionId))

Answer 3

在SQL中，我会将JOIN和GROUP BY结合起来找到每本书的最新版本：

span {
    color: purple;
}

以下LINQ查询将由Entity Framework翻译成完全相同的内容：

<p>I will display <span>&#x2714;<span><p>

EF Linq获取表

3 个答案: