将带有 LEFT JOIN 的 SQL 转换为 Linq（方法语法）

Question

这是我在这里的第一篇文章，所以如果我有任何问题，请告诉我，我会修复它。

我正在努力将带有左连接的简单 SQL 语句转换为 LINQ 语句（方法语法）。我不能使用 Linquer，因为这是一个 .Net Core 5.0 MVC 项目。

假设我有两个表：

dbo.OrganisationChannel (Id, OrganisationId, ChannelId)
dbo.Channel (Id, ChannelName, ChannelUrl)

我想显示一个组织目前没有的所有频道。

这是正确的 SQL 查询

SELECT c.Id,  c.ChannelName, c.ChannelUrl
FROM dbo.Channel c
LEFT JOIN dbo.OrganisationChannel oc ON  c.Id = oc.ChannelId
WHERE oc.ChannelId IS NULL OR oc.OrganisationId <> 1

但是，对应的.GroupJoin 和.SelectMany 让我很困惑。我找不到合适的地方添加 WHERE 子句：

var groupItems = db.Channel
    .GroupJoin(
        db.OrganisationChannel,
        c => c.Id,
        oc => oc.ChannelId,
        (c, oc) => new { c, oc })
    .SelectMany(
        x => x.oc.DefaultIfEmpty(),
        (chan, orgChan) => new
        {
            Id = chan.c.Id,
            ChannelName = chan.c.ChannelName,
            ChannelUrl = chan.c.ChannelUrl,
            IsActive = chan.c.IsActive,
        }
        );

如果能得到任何帮助，我将不胜感激，谢谢！

硅

Answer 1

带有 LEFT JOIN 的方法语法是一场噩梦。如果您真的想要方法语法，请安装 Reshaper 并单击“转换为方法链”。但我不建议这样做 - 查询变得不可维护。

查询语法很简单

var query = 
   from c in db.Channel
   join oc in db.OrganisationChannel on c.Id equals oc.ChannelId into gj
   from oc in gj.DefaultIfempty()
   where (int?)oc.ChannelId == null || oc.OrganisationId != 1
   select new 
   {
       c.Id,
       c.ChannelName, 
       c.ChannelUrl
   };

Answer 2

您可以使用LeftJoin 扩展方法：

public static IQueryable<TResult> LeftJoin<TResult, Ta, Tb, TKey>(this IQueryable<Ta> TableA, IEnumerable<Tb> TableB, Expression<Func<Ta, TKey>> outerKeySelector, Expression<Func<Tb, TKey>> innerKeySelector, Expression<Func<JoinIntermediate<Ta, Tb>, Tb, TResult>> resultSelector)
{
    return TableA.GroupJoin(TableB, outerKeySelector, innerKeySelector, (a, b) => new JoinIntermediate<Ta, Tb> { Value = a, ManyB = b }).SelectMany(intermediate => intermediate.ManyB.DefaultIfEmpty(), resultSelector);
}

public class JoinIntermediate<Ta, Tb>
{
    public Ta Value { get; set; }

    internal IEnumerable<Tb> ManyB { get; set; }
}

它的用法类似于Join 扩展方法，但将执行左连接而不是常规连接。然后，您可以在调用 LeftJoin 之后立即将调用添加到 Where 方法。

Answer 3

使用以下查询代替 lambda 表达式

            from left in lefts
            join right in rights on left equals right.Left into leftRights
            from leftRight in leftRights.DefaultIfEmpty()
            select new { }

Answer 4

查看此网址https://dotnettutorials.net/lesson/left-outer-join-in-linq/

也是我的工作代码示例：

 UserApiKeys
            .Where(w => w.AppID == AppID && w.IsActive)
            .Join(
                    UserApiApplications,
                    keys => keys.AppID,
                    apps => apps.AppID,
                    (keys, apps) => new { UserApiKeys = keys, UserApiApplications = apps}
                )
                .OrderByDescending(d => (d.UserApiKeys.ExpirationDate ?? DateTime.MaxValue))
                .Select(s => new {
                    ApiKey = s.UserApiKeys.ApiKey, 
                    IsActive = s.UserApiKeys.IsActive, 
                    SystemName = s.UserApiKeys.SystemName,
                    ExpirationDate = (s.UserApiKeys.ExpirationDate == null)
                                                                ? "Newer Expires"
                                                                : s.UserApiKeys.ExpirationDate.ToString(),
                    s.UserApiApplications
                })
                .ToList()

另外，参考@nalka关于扩展方法使用的帖子：

NotificationEvents
.Where(w => w.ID == 123)
.LeftJoin(
        Events, 
        events => events.EventID, ev => ev.EventID, 
        (events, ev) => new { NotificationEvents = events, Events = ev }
);