關鍵字： OrderBy 、 GroupBy 、 Into 、 Count 、 Max 、 Min 、 Average 、 Sum 、 Aggregate
OrderBy
OrderBy是用來將結果集做昇冪排序，加上 descending表示降冪排序。
var query1 = from emp in Employees
orderby emp.FirstName, emp.LastName, emp.City descending
select new
{
emp.EmployeeId,
emp.Age,
emp.FirstName
};
若是使用 Lambda 表示式，當有多個欄位做排序時，必須使用 ThenBy或 ThenByDescending。
var query2 = Employees
.OrderBy(emp => emp.FirstName)
.ThenBy(emp => emp.LastName)
.ThenByDescending(emp => emp.City)
.Select(emp => new
{
emp.EmployeeId,
emp.Age,
emp.FirstName
});
GroupBy
GroupBy是用來將結果集分組。
簡單形式
var query1 =
from order in OrderDetails
where order.OrderID == 10248 || order.OrderID == 10249
group order by order.OrderID into gorder
select gorder;
由上面 LINQPad 所得到的回傳結果， 看的出來，GroupBy只是將結果依 GroupBy 所指定的 Key 值區分成不同的組別而已。 若要列舉這些結果可以這麼用：
foreach (var gorder in query1)
{
myDebug.WriteLine("Key : {0}", gorder.Key);
foreach (var order in gorder)
{
myDebug.WriteLine("{0} {1} {2}", order.ProductID, order.Quantity, order.UnitPrice);
}
}
若使用 Lambda 可以這麼操作：
var query2 = OrderDetails
.Where(order => ((order.OrderID == 10248) || (order.OrderID == 10249)))
.GroupBy(order => order.OrderID)
.Select(gorder => gorder);
多重欄位分組(Multiple Columns)
若要分組多個欄位，必須使用匿名型別。
group x by new { x.Column1, x.Column2 }
它的意義是說，每個 Key 值是由二個欄位組成，而這個 Key 值本身也是個匿名型別。
例如：
var query3 =
from order in OrderDetails
where order.OrderID == 10285 || order.OrderID == 10294
group order by new { order.OrderID, order.ProductID };
foreach (var gorder in query3)
{
myDebug.WriteLine("Key : OrderID:{0} ProductID:{1}", gorder.Key.OrderID, gorder.Key.ProductID);
foreach (var order in gorder)
{
myDebug.WriteLine("{0} {1} {2}", order.ProductID, order.Quantity, order.UnitPrice);
}
}
使用運算式分組(Expression)
通常使用上大都依據欄位值來分組，但是這裡也可以使用運算式分組。
下面這個例子，會依據價格是否大於 20 來分組
from order in OrderDetails
where order.OrderID==10285 || order.OrderID==10294
group order by new { UnitPrice = order.UnitPrice > 20 } into g
select g
GroupBy + OrderBy
依群組 Key 值欄位排序
//eg1
from order in OrderDetails
where order.OrderID==10285 || order.OrderID==10294
group order by order.OrderID into g
orderby g.Key
select g
//eg2
from order in OrderDetails
where order.OrderID==10285 || order.OrderID==10294
group order by new { order.OrderID, order.ProductID } into g
orderby g.Key.OrderID, g.Key.ProductID
select g
依群組中的資料排序
下面範例，群組的排序是依據每一個組別中的第一筆的ProductID順序。
from order in OrderDetails
where order.OrderID==10285 || order.OrderID==10294
group order by order.OrderID into g
orderby g.First().ProductID
select g
Into
into關鍵字可以用於建立暫時的識別項，用來識別 group、 join 或 select 子句的結果。
範例：
from order in OrderDetails
where order.OrderID==10285 || order.OrderID==10294
group order by order.OrderID into g
orderby g.Key
select g
Count 、 Max 、 Min 、 Average 、 Sum 、 Aggregate
Aggregation 包含 Max 、 Min 、 Average 、 Sum 、 Count 、 Aggregate 等關鍵字。
使用這一類功能時，都會使用立即查詢，而不是延遲查詢。
Count
Count與 LongCount都可以取得元素個數。返回型別不同而已。
簡單形式
var result1 =
(
from order in OrderDetails
where order.OrderID == 10285 || order.OrderID == 10294
group order by order.OrderID into gorder
select gorder
).Count();
myDebug.WriteLine("Result : {0}", result1.ToString());
var result2 = OrderDetails
.Where(order => ((order.OrderID == 10285) || (order.OrderID == 10294)))
.GroupBy(order => order.OrderID)
.Count();
分組統計資料
from order in OrderDetails
group order by order.OrderID into g
select new
{
g.Key,
RowCount = g.Count()
}
OrderDetails
.GroupBy (order => order.OrderID)
.Select ( g => new
{
Key = g.Key,
RowCount = g.Count ()
})
帶條件分組統計
相檔於 SQL 中的 Having
from order in OrderDetails
group order by order.OrderID into g
where g.Count() > 5
select new
{
g.Key,
RowCount = g.Count()
}
OrderDetails
.GroupBy (order => order.OrderID)
.Where (g => g.Count () > 5)
.Select ( g => new
{
Key = g.Key,
RowCount = g.Count ()
})
Max 、 Min
簡單形式
取得 Products 中最貴的價格
//方法１
(
from p in Products
select p.UnitPrice
).Max()
//方法２
(
from p in Products
select p
).Max(x=>x.UnitPrice)
//方法１
Products.Max(x=>x.UnitPrice)
//方法２
Products.Select(x=>x.UnitPrice).Max()
找整筆資料
取得 Products 中最貴的產品
(
from p in Products
where p.UnitPrice ==
(
from mp in Products
select mp.UnitPrice
).Max()
select p
).Take(1)
Products.First(
p => p.UnitPrice == Products.Max(x=>x.UnitPrice)
)
分組統計資料
取得每一張 Order 中最貴的產品
from order in OrderDetails
group order by order.OrderID into g
select new
{
g.Key,
Product = from details in g
where details.UnitPrice == g.Max(detail => detail.UnitPrice)
select details
}
OrderDetails
.GroupBy (order => order.OrderID)
.Select ( g => new
{
Key = g.Key,
Product = g.Where (details => details.UnitPrice == g.Max (detail => detail.UnitPrice))
}
)
Average 、 Sum
簡單形式
取得 Products 中的平均價格
(
from p in Products
select p.UnitPrice
).Average()
(
from p in Products
select p
).Average(x=>x.UnitPrice)
Products.Average(x=>x.UnitPrice)
Products.Select(x=>x.UnitPrice).Average()
分組統計資料
取得每一張 Order 的平均價格
from order in OrderDetails
group order by order.OrderID into g
select new
{
g.Key,
AveragePrice = g.Average (detail => detail.UnitPrice)
}
OrderDetails
.GroupBy (order => order.OrderID)
.Select ( g => new
{
Key = g.Key,
AveragePrice = g.Average (detail => detail.UnitPrice)
})
Aggregate
累計