如何創建Linq2Sql查詢，該查詢將對鏈接表中的記錄進行分組並計算2個計數字段

Question

在這里，我發現了如何使用Linq2Sql聯接表並計算鏈接記錄LINQ的數量-左聯接，分組依據和計數

我已經實現了它，對我來說效果很好：以下表達式

var v = from c in db.GetTable<Country>()
        join t0 in db.GetTable<Team>() on c.Id equals t0.CountryId into t1
        from team in t1.DefaultIfEmpty()
        group team by c.Id into teamsGrouped
        select new CountryTeamsInfo
            {
                CountryId = teamsGrouped.Key,
                TeamsTotal = teamsGrouped.Count(),
                // TeamsWithoutOwnerFree = teamsGrouped.Count(t => t.OwnerId==0)
            }
            ;
         List<CountryTeamsInfo> res = v.ToList();

生成以下查詢：

SELECT c.Id, Count(*) as c1
FROM countries c
LEFT JOIN teams t1 ON c.Id = t1.Country
GROUP BY c.Id 

實際上，我還需要計算OwnerId字段不等於0的那些鏈接記錄。

看來我應該在linq表達式中取消注釋該行（TeamsWithoutOwnerFree = teamsGrouped.Count（t => t.OwnerId == 0）），但這不起作用，嘗試執行會導致錯誤：

給定的鍵在字典中不存在

該查詢不會進入SQL日志文件，並且我無法在調試器中對其進行檢查。

什么是從“團隊”表計算符合附加條件的行的正確方法？

PS，如果有關系，我使用C＃4.0，MySql 5.1和BLToolkit 4.1

Answer 1

也許嘗試使用GroupJoin()進行查詢：

var result = db.GetTable<Country>()
               .GroupJoin(db.GetTable<Team>(),
                   c => c.Id,
                   t => t.CountryId,
                   (country, teams) => new
                   {
                       CountryId = country.Id,
                       TeamsTotal = teams.Count(),
                       TeamsWithoutOwnerFree = teams.Count(t => t.OwnerId == 0)
                   })
               .ToList();

Answer 2

由於RePierre的幫助，我得以找到正確的查詢：

var v = db.GetTable<Country>().Where(country => country.Allowed)
                    .GroupJoin(
                        db.GetTable<Team>(),
                        country => country.Id,
                        team => team.CountryId,
                        (country, teams) => new CountryTeamsInfo
                                                {
                                                    CountryId = country.Id,
                                                    TeamsTotal = teams.Count(),
                                                    TeamsWithoutOwnerFree = teams.Count(t => t.OwnerId != 0),
                                                }
                    ).GroupJoin(
                        db.GetTable<Team>().Where(te=>te.OwnerId==0),
                        cti => cti.CountryId,
                        team => team.CountryId,
                        (cti, teams) => new CountryTeamsInfo
                        {
                            CountryId = cti.CountryId,
                            TeamsTotal = cti.TeamsTotal,
                            TeamsWithoutOwnerFree = teams.Count(t => t.OwnerId != 0),
                        }
                    )
                    ;

我擔心的是它使用2個子查詢...將嘗試對其進行優化。 任何想法都很棒

PS實際上，生成的SQL查詢看起來也很丑陋：

SELECT
cti.Id as Id1,
cti.c1 as c11,
(
    SELECT
        Count(*)
    FROM
        teams te
    WHERE
        cti.Id = te.Country AND te.User = 0
) as c2
FROM
(
    SELECT
        country.Id,
        (
            SELECT
                Count(*)
            FROM
                teams t1
            WHERE
                country.Id = t1.Country
        ) as c1
    FROM
        countries country
    WHERE
        country.allow

Answer 3

這是在我的環境中有效的查詢：

from c in db.GetTable<Country>()
where c.Allowed
select new CountryTeamsInfo
{
    CountryId = c.Id,
    TeamsTotal = db.GetTable<Team>().Count(t => t.CountryId == c.Id && t.Allowed),
    TeamsHasOwner = db.GetTable<Team>().Count(t => t.CountryId == c.Id && t.Allowed && t.OwnerId != 0),
}

不是最好的解決方案（我需要重復團隊選擇標准t。在每個子查詢中都允許），但是仍然可以很好地工作，並且生成的SQL很好。