LINQ 3个内连接和1个左外连接

Question

想知道LINQ为什么没有Left Join方法。 我一直在尝试用SO上的无数示例来解决这个问题，但是没有运气。 其他示例显示一个连接的简单示例。 如果将联接分组，则只能在select语句中获得对TradeCountries表的引用。

作为LINQ的新手，我本可以在4个小时前用一个简单的SELECT语句完成此操作，但是在这里，我试图弄清楚为什么LeftJoin方法不包含在LINQ中。

为了使这项工作有效，需要更改带有“ LeftJoin”的行吗？

/* 
* GetTop5Distributors 
@param  int array of series IDs
*/
public List<TopDistributors> Get5TopDistributors(IEnumerable<int> seriesIds)
{
    _context = new MySQLDatabaseContext();
    var result = _context.TradesTrades
.Join(_context.TradesSeries, tt => tt.SeriesId, ts => ts.Id, (tt, ts) => new { tt, ts })
.Join(_context.TradesTradeDistributors, tsd => tsd.tt.Id, ttd => ttd.TradeId,
    (tsd, ttd) => new { tsd, ttd })
.Join(_context.TradesOrganisations, tsdto => tsdto.ttd.DistributorId, to => to.Id,
    (tsdto, to) => new { tsdto, to })
.LeftJoin(_context.TradesCountries, tsdc => tsdc.to.CountryId, tc => tc.Id, 
    (tsdc, tc) => new {tsdc, tc})
.Where(x => seriesIds.Contains(x.tsdc.tsdto.tsd.tt.SeriesId))
.Where(x => x.tsdc.tsdto.tsd.tt.FirstPartyId == null)
.Where(x => x.tsdc.tsdto.tsd.tt.Status != "closed")
.Where(x => x.tsdc.tsdto.tsd.tt.Status != "cancelled")
.GroupBy(n => new { n.tsdc.tsdto.tsd.tt.SeriesId, n.tsdc.tsdto.ttd.DistributorId })
.Select(g =>
    new TopDistributors
    {
        SeriesId = g.Key.SeriesId,
        DistributorName = g.Select(i => i.tsdc.to.Name).Distinct().First(),
        IsinNickname = g.Select(i => i.tsdc.tsdto.tsd.ts.Nickname).Distinct().First(),
        CountryName = g.Select(i => i.tc.Name).Distinct().First(),
        CommissionTotal = Math.Ceiling(g.Sum(i => i.tsdc.tsdto.ttd.Commission))
    }
)
.OrderByDescending(x => x.CommissionTotal)
.Take(5)
.ToList();

    return result;
}

这是相当简单的select语句，需要花费很长时间或数量级才能转换为LINQ。

SELECT
trades_trades.series_id,
trades_organisations.`name`,
trades_series.nickname,
trades_countries.name as Country_Name,
SUM(trades_trade_distributors.commission) as Commission_Total
FROM
trades_trades
JOIN trades_series
ON trades_series.id = trades_trades.series_id
JOIN trades_trade_distributors
ON trades_trades.id = trades_trade_distributors.trade_id
JOIN trades_organisations
ON trades_trade_distributors.distributor_id = trades_organisations.id
LEFT JOIN trades_countries
ON trades_organisations.country_id = trades_countries.id
WHERE trades_trades.series_id   IN (
    17,
    18)
    AND trades_trades.first_party_id IS NULL
    AND trades_trades.status <> 'closed'
    AND trades_trades.status <> 'cancelled'
GROUP BY trades_trades.series_id, trades_trade_distributors.distributor_id
ORDER BY Commission_Total DESC

Answer 1

按照我的食谱，这是将SQL或多或少直接转换为LINQ的方法。 我搬到了where是附近什么约束，并用let为营造一个方便的名字Sum ，如LINQ不允许你参考匿名对象成员转发。

var ans = from tt in trades_trades
          where new[] { 17, 18 }.Contains(tt.series_id) && tt.first_party_id == null &&
                tt.status != "closed" && tt.status != "cancelled"
          join ts in trades_series on tt.series_id equals ts.id
          join ttd in trades_trade_distributors on tt.id equals ttd.trade_id
          join to in trades_orginizations on ttd.distributor_id equals to.id
          join tc in trades_countries on to.country_id equals tc.id into tcj
          from tc in tcj.DefaultIfEmpty() // GroupJoin -> left join
          group new { tt, ts, ttd, to, tc } by new { tt.series_id, ttd.distributor_id } into tradeg
          let Commission_Total = tradeg.Sum(trade => trade.ttd.commission)
          orderby Commission_Total descending
          select new {
            tradeg.Key.series_id,
            tradeg.First().to.name,
            tradeg.First().ts.nickname,
            Country_Name = tradeg.First().tc == null ? null : tradeg.First().tc.name,
            Commission_Total
          };