Sql server 对Lambda表达式C的SQL查询#

SQL查询：

SELECT M.MailItemId, m.Subject, ISNULL(m.SendCC, ''), ISNULL(attachments.counter, 0) Counters, M.CreationDate
FROM MailItem AS M
LEFT OUTER JOIN (
    SELECT MailItemId, COUNT(MailItemId) counter
    FROM Attachment Group By MailItemId
) AS attachments ON M.MailItemId = attachments.MailItemId

有两种模式：邮件项目和附件。 MailItemId是MailItem中的主键，是附件模型中的外键。

---

要将上述查询转换为lambda表达式。

因此您有一个包含

邮件项的表和一个包含
附件的表。在
邮件项目
和
附件
之间有一个简单的一对多关系：每个
邮件项目
都有零个或多个
附件
，每个
附件
都只属于一个
邮件项目
，即外键所指的
邮件项目

通常，人们想要“带有附件的邮件”，他们不想要左外连接：


邮件项目1及附件A、C
邮件项目2及附件B、D、F
无附件的邮件项目3
邮件项目4及附件E

另一方面，您似乎更喜欢以下结果：

MailItem Attachment
    1        A
    2        B
    1        C
    3      <null>
    4        E
    2        D
    2        F

左外连接
使用虚拟ICollections
如果您遵循了实体框架约定，您将拥有类似于以下内容的类：

class MailItem
{
    public int Id {get; set; }
    ...

    // every MailItem has zero or more Attachments (one-to-many)
    public virtual ICollection<Attachment> Attachments {get; set;}
}

class Attachment
{
    public int Id {get; set; }
    ...

    // every Attachment belongs to exactly one MailItem, using foreign key:
    public int MailItemId {get; set;}
    public virtual MailItem MailItem {get; set;}
}

实体框架知道这种关系，并将为您执行正确的（组）连接

我觉得这看起来比GroupJoin更自然，无论后面是否跟有selectMany，请不要撤消编辑。。。没有格式化就无法描述。到目前为止，你用Linq做了什么尝试？向我们展示您的代码和实体模型等。。
var result = dbContext.MailItems.GroupJoin(dbContext.Attachments,
    mailitem => mailItem.MailItemId,
    attachment => attachment.MailItemId,
    (mailItem, attachmentsOfThisMailItem) => new
    {
         MailItem = mailItem,
         Attachments = attachmentsOfThisMailItem,
    })

    // make it a left outer join, using SelectMany
    .SelectMany(groupJoinResult => groupJoinResult.Attachments,

    (groupJoinResult, attachment) => new
    {
        MailId = groupJoinResult.MailItem.Id,
        Sender = groupJoinResult.MailItem.Sender,
        ...

        AttachmentId = attachment.Id,
        Format = attachment.Format,
        ...
    });

class MailItem
{
    public int Id {get; set; }
    ...

    // every MailItem has zero or more Attachments (one-to-many)
    public virtual ICollection<Attachment> Attachments {get; set;}
}

class Attachment
{
    public int Id {get; set; }
    ...

    // every Attachment belongs to exactly one MailItem, using foreign key:
    public int MailItemId {get; set;}
    public virtual MailItem MailItem {get; set;}
}

var mailItemsWithTheirAttachments = dbContext.MailItems.Select(mailItem => new
{
    Id = mailItem.Id,
    Sender = mailItem.Sender,
    ...

    Attachments = mailItem.Attachments
        .Where(attachment => ....)     // only if you don't want all Attachments
        .Select(attachment => new
        {
            Id = attachment.Id,
            ...
        })
        .ToList(),
});