使用 LINQ 表达式进行多个左连接并使用 ISNULL 功能

Question

我正在工作中构建步数跟踪网络应用。我正在使用最新的 EF Core。我正在与之交互的三个表：

1. wg：WellnessGroup（WellnessGroupId，名称）
2. wgu：WellnessGroupUser（查表：WellnessGroupId、EmployeeId）
3. wsl: WellnessStepsLog (EmployeeId, StepCount)

我想要的是获得所有 WellnessGroups 和该组的总步数。如果该组还没有附加步骤，我希望 NULL 值为 0。我有这个 SQL 语句，它给了我所需的数据：

SELECT wg.Name, SUM(ISNULL(wsl.StepCount, 0)) AS steps
FROM dbo.WellnessGroup AS wg
LEFT JOIN dbo.WellnessGroupUser AS wgu
ON wgu.WellnessGroupId = wg.Id
LEFT JOIN dbo.WellnessStepsLog AS wsl
ON wsl.EmployeeId = wgu.AzureAdUserId
GROUP BY wg.Name
ORDER BY steps DESC;

我已经设法将 2 个 LINQ 表达式一起放在我的控制器上，这只会给我提供与它们相关联的步骤的 WellnessGroup，如果没有步骤，则不会给我 WellnessGroup 数据：

 var query = _dbContext.WellnessGroupUser
                    .Include(x => x.WellnessGroup)
                .Join(_dbContext.WellnessStepsLog, group => 
                group.AzureAdUserId, steps => steps.EmployeeId,
                    (group, steps) => new
                    {
                        Steps = steps.StepCount,
                        Date = steps.TrackedDate,
                        Group = group.WellnessGroup.Name

                    }).Where(x => x.Date >= yearToDate).Where(x => x.Date <= endDate);

 var stepsByGroup = query
                .GroupBy(x => x.Group)
                .Select(s => new
                {
                    Group = s.Key,
                    Date = s.Max(x => x.Date),
                    Steps = s.Sum(x => x.Steps)
                });

Answer 1

一种方法是查询所有 WellnessGroup 并在其中构建总和作为第二个子查询。像这样：

var query =
    db.WellnessGroup.Select(wg => new {
        wg.WellnessGroupId,
        sum = (int?) wg.WellnessGroupUser
                       .Sum(wgu => wgu.Employee.WellnessStepsLog.Sum(wsl => wsl.StepCount))
    });

请注意，转换为 (int?) 很重要。否则 sum 被假定为 int，如果一行没有 sum，则会导致 InvalidOperationException。

另一种方法是先建立所有的总和。然后与 WellnessGroups 进行外部连接：

// sum up all stepcounts
var q1 =
    from wgu in db.WellnessGroupUser
    from wsl in db.WellnessStepsLog
    where wgu.EmployeeId == wsl.EmployeeId
    group wsl.StepCount by wgu.WellnessGroupId
    into g
    select new {WellnessGroupId = g.Key, Sum = g.Sum()};

// join with all WellnessGroups
var q2 =
    from wg in db.WellnessGroup
    join s in q1 on wg.WellnessGroupId equals s.WellnessGroupId into sj
    from sum in sj.DefaultIfEmpty()
    select new {wg, sum = (int?) sum.Sum};

---

编辑：

由于 OP 后来在 cmets 中询问如何按多个字段进行分组。这是一个按WellnessGroup.WellnessGroupId 和WellnessStepsLog.TrackedDate 月份分组的示例。通过将它们放在new { ... } 中，GroupBy 中可以有多个字段。因此，第一个查询为每个可能的 WellnessGroup / Month 组合创建一条线。第二个查询与之前一样执行 WellnessGroup 的外连接：

var q1 =
    from wgu in db.WellnessGroupUser
    from wsl in db.WellnessStepsLog
    where wgu.EmployeeId == wsl.EmployeeId
    group wsl.StepCount by new { wgu.WellnessGroupId, wsl.TrackedDate.Month }
    into g
    select new {g.Key.WellnessGroupId, g.Key.Month, Sum = g.Sum()};

// join with all WellnessGroups
var q2 =
    from wg in db.WellnessGroup
    join s in q1 on wg.WellnessGroupId equals s.WellnessGroupId into sj
    from sum in sj.DefaultIfEmpty()
    select new {wg.WellnessGroupId, Month = (int?) sum.Month, Sum = (int?) sum.Sum};