Linq 用法笔记

Linq中怎么用 between…and?

 var query = from p in context.Parent
            from c in context.Child.Where(x => p.cob >= x.effective)
                                   .Where(x => p.cob <= x.expiry)
                                   .DefaultIfEmpty()
            group p by p.cob into pg
            select new
            {
              cob = pg.Key,
              count = pg.Count()
            };

下面这个是用多个where条件来处理between…and，但是是内连接。

var appointmentNoShow = from a in appointments
                        from p in properties
                        from c in clients
                        where a.Id == p.OID
                        where a.Start.Date >= startDate.Date
                        where a.Start.Date<=endDate.Date

怎么用 in ？

var allowedStatus = new[]{ "A", "B", "C" };
var filteredOrders = orders.Order.Where(o => allowedStatus.Contains(o.StatusCode));

or in query syntax:

 var filteredOrders = from order in orders.Order
                     where allowedStatus.Contains(order.StatusCode)
                     select order;

怎么使用多个连接条件 join...on ？

var query = from obj_i in set1
join obj_j in set2 on
    new {
      JoinProperty1 = obj_i.SomeField1,
      JoinProperty2 = obj_i.SomeField2,
      JoinProperty3 = obj_i.SomeField3,
      JoinProperty4 = obj_i.SomeField4
    }
    equals
    new {
      JoinProperty1 = obj_j.SomeOtherField1,
      JoinProperty2 = obj_j.SomeOtherField2,
      JoinProperty3 = obj_j.SomeOtherField3,
      JoinProperty4 = obj_j.SomeOtherField4
    }
http://*.com/questions/3020442/linq-joining-in-c-sharp-with-multiple-conditions

如果要达到这个sql的效果：

SELECT * FROM table1 a
LEFT JOIN table2 b ON a.col1 = b.key1 AND
a.col2 = b.key2 AND
b.from_date <= now() AND
b.deleted = 0;

可以这样：

var query = (from x in context.table1
             join y in context.table2 on
             new  {
                  Key1 = x.col1,
                  Key2 = x.col2
                  Key3 = true,
                  Key4 = true
                  } equals
     {
        Key1 = b.key1,
        Key2 =  b.key2,
        Key3 = b.from_date< DateTime.Now,
        Key4 = !b.deleted
     }
             into result
from r in result.DefaultIfEmpty()
select new  {x.Something, r.Something}
http://*.com/questions/7765230/linq-to-entity-multiple-join-conditions

怎么使用group ... by 分组？

如sql：

SELECT * FROM <TableName> GROUP BY <Column1>,<Column2>

group x by new { x.Column1, x.Column2 }
或者：
.GroupBy(x => new { x.Column1, x.Column2 })

var query = (from t in Transactions
             group t by new {t.MaterialID, t.ProductID}
             into grp
                    select new
                    {
                        grp.Key.MaterialID,
                        grp.Key.ProductID,
                        Quantity = grp.Sum(t => t.Quantity)
                    }).ToList();