在連接包含周期性事件的兩個表的LINQ查詢方面需要幫助

Question

我的現實生活中的例子太晦澀難解，無法解釋，但這與我正在嘗試做的非常相似...

Month表具有列： Id ， Name

Holiday表具有以下列： Id ， MonthId ， DayOfMonth ， Name

Appointment表具有以下列： Id ， MonthId ， DayOfMonth ， Description

如何生成按月和月日排序的獨特事件（節假日和約會）列表？

樣本結果：

Month  Holiday     Day  Appointment  Day
----------------------------------------
Nov                     Fly to NYC   25
Nov    T-Giving    26
Nov                     Fly home     29
Dec    Xmas        25

因此，我想為假期和活動使用單獨的列，但我希望它們都是唯一的並按月日順序列出。

這是我到目前為止的內容（請參閱嵌入式注釋）：

var events =
    from
        m in GetMonths()
    join
        h in GetHolidays()
        on m.Id equals h.MonthId
    join
        a in GetAppointments()
        on m.Id equals a.MonthId
    where
        //something that narrows all combinations of events to only unique events
    orderby
        m.Id,
        // something that interleaves h.DayOfMonth with m.DayOfMonth
    select
        new
        {
            Month = m.Name,
            Holiday = h.Name,
            HolidayDay = h.DayOfMonth,
            Appointment = a.Description,
            AppointmentDay = a.DayOfMonth
        };

Answer 1

這是一個使用UNION而不是LEFT OUTER的替代答案，它完全返回您要查找的結果集（我認為我的第一個答案不能完全滿足您的“獨特”要求）：

var a = from m in month
        join h in holiday on m.Id equals h.MonthId
        select new
        {
            MonthId = m.Id,
            Month = m.Name,
            Holiday = h.Name,
            HolidayDay = h.DayOfMonth,
            Appointment = "",
            AppointmentDay = 0

        };

var b = from m in month
        join p in appointments on m.Id equals p.MonthId
        select new
        {
            MonthId = m.Id,
            Month = m.Name,
            Holiday = "",
            HolidayDay = 0,
            Appointment = p.Description,
            AppointmentDay = p.DayOfMonth
        };

var events = from o in a.Union(b)
            orderby o.MonthId, o.HolidayDay + o.AppointmentDay
            select o;

Answer 2

您需要在SQL中進行LEFT OUTER連接，LINQ看起來像這樣（未經測試）：

var events =
    from
        m in GetMonths()
    groupjoin
        h in GetHolidays()
        on m.Id equals h.MonthId
        into hol = group
    from
        h in hol.DefaultIfEmpty()
    groupjoin
        a in GetAppointments()
        on m.Id equals a.MonthId
        into appt = group
    from
        a in appt.DefaultIfEmpty()
    where
        //something that narrows all combinations of events to only unique events
    orderby
        m.Id,
        // something that interleaves h.DayOfMonth with m.DayOfMonth
    select
        new
        {
            Month = m.Name,
            Holiday = h.Name,
            HolidayDay = h.DayOfMonth,
            Appointment = a.Description,
            AppointmentDay = a.DayOfMonth
        };