HOW TO：分組序列中的項目 (LINQ to SQL)
更新： November 2007
GroupBy 運算子會分組序列中的項目。下列範例使用 Northwind 資料庫。
.gif "注意事項") 注意事項： |
|---|
GroupBy 查詢中的 Null 資料行值有時會擲回 InvalidOperationException。如需詳細資訊，請參閱疑難排解 (LINQ to SQL) 的＜GroupBy InvalidOperationException＞一節。 |
範例
下列範例會根據 CategoryID 來分割 Products。
Dim prodQuery = From prod In db.Products _
Group prod By prod.CategoryID Into grouping = Group
For Each grp In prodQuery
Console.WriteLine(vbNewLine & "CategoryID Key = {0}:", _
grp.CategoryID)
For Each listing In grp.grouping
Console.WriteLine(vbTab & listing.ProductName)
Next
Next
IQueryable<IGrouping<Int32?, Product>> prodQuery =
from prod in db.Products
group prod by prod.CategoryID into grouping
select grouping;
foreach (IGrouping<Int32?, Product> grp in prodQuery)
{
Console.WriteLine("\nCategoryID Key = {0}:", grp.Key);
foreach (Product listing in grp)
{
Console.WriteLine("\t{0}", listing.ProductName);
}
}
下列範例使用 Max 來找出每個 CategoryID 的最高單價。
Dim query = From p In db.Products _
Group p By p.CategoryID Into g = Group _
Select CategoryID, MaxPrice = g.Max(Function(p) p.UnitPrice)
var q =
from p in db.Products
group p by p.CategoryID into g
select new
{
g.Key,
MaxPrice = g.Max(p => p.UnitPrice)
};
下列範例使用 Average 來找出每個 CategoryID 的平均 UnitPrice。
Dim q2 = From p In db.Products _
Group p By p.CategoryID Into g = Group _
Select CategoryID, AveragePrice = g.Average(Function(p) _
p.UnitPrice)
var q2 =
from p in db.Products
group p by p.CategoryID into g
select new
{
g.Key,
AveragePrice = g.Average(p => p.UnitPrice)
};
下列範例使用 Sum 來找出每個 CategoryID 的總 UnitPrice。
Dim priceQuery = From prod In db.Products _
Group prod By prod.CategoryID Into grouping = Group _
Select CategoryID, TotalPrice = grouping.Sum(Function(p) _
p.UnitPrice)
For Each grp In priceQuery
Console.WriteLine("Category = {0}, Total price = {1}", _
grp.CategoryID, grp.TotalPrice)
Next
var priceQuery =
from prod in db.Products
group prod by prod.CategoryID into grouping
select new
{
grouping.Key,
TotalPrice = grouping.Sum(p => p.UnitPrice)
};
foreach (var grp in priceQuery)
{
Console.WriteLine("Category = {0}, Total price = {1}",
grp.Key, grp.TotalPrice);
}
下列範例使用 Count 來尋找每個 CategoryID 中停產的 Products 數。
Dim disconQuery = From prod In db.Products _
Group prod By prod.CategoryID Into grouping = Group _
Select CategoryID, NumProducts = grouping.Count(Function(p) _
p.Discontinued)
For Each prodObj In disconQuery
Console.WriteLine("CategoryID = {0}, Discontinued# = {1}", _
prodObj.CategoryID, prodObj.NumProducts)
Next
var disconQuery =
from prod in db.Products
group prod by prod.CategoryID into grouping
select new
{
grouping.Key,
NumProducts = grouping.Count(p => p.Discontinued)
};
foreach (var prodObj in disconQuery)
{
Console.WriteLine("CategoryID = {0}, Discontinued# = {1}",
prodObj.Key, prodObj.NumProducts);
}
下列範例使用後續 where 子句，來尋找所有至少有 10 個產品的類別。
Dim prodCountQuery = From prod In db.Products _
Group prod By prod.CategoryID Into grouping = Group _
Where grouping.Count >= 10 _
Select CategoryID, ProductCount = grouping.Count
For Each prodCount In prodCountQuery
Console.WriteLine("CategoryID = {0}, Product count = {1}", _
prodCount.CategoryID, prodCount.ProductCount)
Next
var prodCountQuery =
from prod in db.Products
group prod by prod.CategoryID into grouping
where grouping.Count() >= 10
select new
{
grouping.Key,
ProductCount = grouping.Count()
};
foreach (var prodCount in prodCountQuery)
{
Console.WriteLine("CategoryID = {0}, Product count = {1}",
prodCount.Key, prodCount.ProductCount);
}
下列範例會根據 CategoryID 和 SupplierID 來分組產品。
Dim prodQuery = From prod In db.Products _
Group prod By Key = New With {prod.CategoryID, prod.SupplierID} _
Into grouping = Group
For Each grp In prodQuery
Console.WriteLine(vbNewLine & "CategoryID {0}, SupplierID {1}", _
grp.Key.CategoryID, grp.Key.SupplierID)
For Each listing In grp.grouping
Console.WriteLine(vbTab & listing.ProductName)
Next
Next
var prodQuery =
from prod in db.Products
group prod by new
{
prod.CategoryID,
prod.SupplierID
}
into grouping
select new { grouping.Key, grouping };
foreach (var grp in prodQuery)
{
Console.WriteLine("\nCategoryID {0}, SupplierID {1}",
grp.Key.CategoryID, grp.Key.SupplierID);
foreach (var listing in grp.grouping)
{
Console.WriteLine("\t{0}", listing.ProductName);
}
}
下列範例會傳回兩個產品序列。第一個序列包含單價低於或等於 10 的產品。第二個序列則包含單價高於 10 的產品。
Dim priceQuery = From prod In db.Products _
Group prod By Key = New With {.Criterion = prod.UnitPrice > 10} _
Into grouping = Group Select Key, grouping
For Each prodObj In priceQuery
If prodObj.Key.Criterion = False Then
Console.WriteLine("Prices 10 or less:")
Else
Console.WriteLine("\nPrices greater than 10")
For Each listing In prodObj.grouping
Console.WriteLine("{0}, {1}", listing.ProductName, _
listing.UnitPrice)
Next
End If
Next
var priceQuery =
from prod in db.Products
group prod by new
{
Criterion = prod.UnitPrice > 10
}
into grouping
select grouping;
foreach (var prodObj in priceQuery)
{
if (prodObj.Key.Criterion == false)
Console.WriteLine("Prices 10 or less:");
else
Console.WriteLine("\nPrices greater than 10");
foreach (var listing in prodObj)
{
Console.WriteLine("{0}, {1}", listing.ProductName,
listing.UnitPrice);
}
}
GroupBy 運算子只能採用單一索引鍵引數。如果需要根據多個索引鍵進行分組，則必須建立匿名型別，如下列範例所示：
Dim custRegionQuery = From cust In db.Customers _
Group cust.ContactName By Key = New With _
{cust.City, cust.Region} Into grouping = Group
For Each grp In custRegionQuery
Console.WriteLine(vbNewLine & "Location Key: {0}", grp.Key)
For Each listing In grp.grouping
Console.WriteLine(vbTab & "{0}", listing)
Next
Next
var custRegionQuery =
from cust in db.Customers
group cust.ContactName by new { City = cust.City, Region = cust.Region };
foreach (var grp in custRegionQuery)
{
Console.WriteLine("\nLocation Key: {0}", grp.Key);
foreach (var listing in grp)
{
Console.WriteLine("\t{0}", listing);
}
}