LINQ中的JOIN和LEFT JOIN等价物

Question

我正在使用以下SQL查询:

SELECT 
a.AppointmentId,
a.Status,
a.Type,
a.Title,
b.Days,
d.Description,
e.FormId
FROM Appointment a (nolock)
LEFT JOIN AppointmentFormula b (nolock)
ON a.AppointmentId = b.AppointmentId and b.RowStatus = 1
JOIN Type d (nolock)
ON a.Type = d.TypeId
LEFT JOIN AppointmentForm e (nolock)
ON e.AppointmentId = a.AppointmentId
WHERE a.RowStatus = 1
AND a.Type = 1
ORDER BY a.Type

我不确定如何在LINQ中实现JOIN.我的所有表都有外键关系.

Answer 1

SELECT A.X, B.Y
FROM A JOIN B ON A.X = B.Y

这个linq方法调用(加入)将生成上面的Join.

var query = A.Join
(
  B,
  a => a.x,
  b => b.y,
  (a, b) => new {a.x, b.y} //if you want more columns - add them here.
);

SELECT A.X, B.Y
FROM A LEFT JOIN B ON A.X = B.Y

这些linq方法调用(对GroupJoin,SelectMany,DefaultIfEmpty)将产生上面的Left Join

var query = A.GroupJoin
(
  B,
  a => a.x,
  b => b.y,
  (a, g) => new {a, g}
).SelectMany
(
  z => z.g.DefaultIfEmpty(),
  (z, b) =>
    new  { x = z.a.x, y = b.y } //if you want more columns - add them here.
);

这里的关键概念是Linq的方法产生分层形状的结果,而不是扁平的行列形状.

• Linq GroupBy生成的结果形式为层次结构,其分组键与元素集合(可能不为空)匹配.SQL的GroupBy子句生成一个具有聚合值的分组键- 没有要使用的子集合.
• 类似地,Linq GroupJoin生成一个分层形状 - 与一组子记录(可能为空)匹配的父记录.Sql LEFT JOIN生成与每个子记录匹配的父记录,如果没有其他匹配则生成空子记录.要从Linq的形状获得Sql的形状,必须使用SelectMany- 解压缩子记录集合- 并使用处理子记录的空集合DefaultIfEmpty.

这是我试图在问题中解释sql:

var query =
  from a in Appointment
  where a.RowStatus == 1
  where a.Type == 1
  from b in a.AppointmentFormula.Where(af => af.RowStatus == 1).DefaultIfEmpty()
  from d in a.TypeRecord //a has a type column and is related to a table named type, disambiguate the names
  from e in a.AppointmentForm.DefaultIfEmpty()
  order by a.Type
  select new { a.AppointmentId, a.Status, a.Type, a.Title, b.Days, d.Description, e.Form }

Answer 2

当我离开袖口时你可能不得不稍微调整一下,但还有几个要记住的重要事项.如果在dbml中正确设置了关系,则应该可以隐式执行内部联接,只需通过初始表访问数据.此外,LINQ中的左连接并不像我们希望的那样直截了当,您必须通过DefaultIfEmpty语法才能实现它.我在这里创建了一个匿名类型,但您可能想要放入DTO类或其他类似的东西.在nulls的情况下我也不知道你想做什么,但你可以使用?? 语法定义一个值,如果值为null,则给出变量.如果您有其他问题,请告诉我......

var query = (from a in context.Appointment
join b in context.AppointmentFormula on a.AppointmentId equals b.AppointmentId into temp
from c in temp.DefaultIfEmpty()
join d in context.AppointmentForm on a.AppointmentID equals e.AppointmentID into temp2
from e in temp2.DefaultIfEmpty()
where a.RowStatus == 1 && c.RowStatus == 1 && a.Type == 1
select new {a.AppointmentId, a.Status, a.Type, a.Title, c.Days ?? 0, a.Type.Description, e.FormID ?? 0}).OrderBy(a.Type);