我有一个表，用于存储产品更改的历史记录，并希望获取Col1或Col2或Col3中有更改的记录列表，但不显示这三个记录中没有任何更改的记录专栏。
这是在SQL中完成的一个示例。你如何对待Linq？
创建用于测试的临时表
CREATE TABLE #ProductHistorical(
IdProductHistorical int IDENTITY(1,1) NOT NULL,
IdProduct int NOT NULL,
DateChange datetime NULL,
Col1 int NOT NULL,
Col2 int NOT NULL,
Col3 int NOT NULL,
CONSTRAINT PK_ProductHistorical PRIMARY KEY CLUSTERED (IdProductHistorical ASC))
GO
插入测试数据
INSERT #ProductHistorical ( IdProduct, DateChange, Col1, Col2, Col3) VALUES (1, CAST(0x0000A13900000000 AS DateTime), 1, 2, 3)
INSERT #ProductHistorical ( IdProduct, DateChange, Col1, Col2, Col3) VALUES (1, CAST(0x0000A13A00000000 AS DateTime), 1, 2, 3)
INSERT #ProductHistorical ( IdProduct, DateChange, Col1, Col2, Col3) VALUES (1, CAST(0x0000A13B00000000 AS DateTime), 1, 2, 3)
INSERT #ProductHistorical ( IdProduct, DateChange, Col1, Col2, Col3) VALUES (1, CAST(0x0000A13C00000000 AS DateTime), 1, 1, 3)
INSERT #ProductHistorical ( IdProduct, DateChange, Col1, Col2, Col3) VALUES (1, CAST(0x0000A13D00000000 AS DateTime), 1, 1, 3)
INSERT #ProductHistorical ( IdProduct, DateChange, Col1, Col2, Col3) VALUES (1, CAST(0x0000A13E00000000 AS DateTime), 2, 2, 2)
INSERT #ProductHistorical ( IdProduct, DateChange, Col1, Col2, Col3) VALUES (1, CAST(0x0000A13F00000000 AS DateTime), 2, 2, 2)
INSERT #ProductHistorical ( IdProduct, DateChange, Col1, Col2, Col3) VALUES (1, CAST(0x0000A14000000000 AS DateTime), 2, 2, 2)
INSERT #ProductHistorical ( IdProduct, DateChange, Col1, Col2, Col3) VALUES (1, CAST(0x0000A14100000000 AS DateTime), 1, 2, 3)
INSERT #ProductHistorical ( IdProduct, DateChange, Col1, Col2, Col3) VALUES (2, CAST(0x0000A14200000000 AS DateTime), 1, 1, 1)
INSERT #ProductHistorical ( IdProduct, DateChange, Col1, Col2, Col3) VALUES (2, CAST(0x0000A14300000000 AS DateTime), 1, 1, 2)
SQL查询
SELECT phWithChanges.DateChange,
phWithChanges.Col1,
phWithChanges.Col2,
phWithChanges.Col3
FROM #ProductHistorical ph
CROSS APPLY (
SELECT TOP 1 *
FROM #ProductHistorical phPost
WHERE phPost.IdProduct=ph.IdProduct AND
phPost.IdProductHistorical>ph.IdProductHistorical AND
(phPost.Col1<>ph.Col1 OR phPost.Col2<>ph.Col2 OR phPost.Col2<>ph.Col2)
ORDER BY phPost.IdProductHistorical ASC) phWithChanges
WHERE ph.IdProduct=1
GROUP BY phWithChanges.DateChange,phWithChanges.Col1,phWithChanges.Col2,phWithChanges.Col3
UNION
--Add First Row
SELECT * FROM
(SELECT TOP 1
phFirst.DateChange,
phFirst.Col1,
phFirst.Col2,
phFirst.Col3
FROM #ProductHistorical phFirst
WHERE phFirst.IdProduct=1 ORDER BY phFirst.IdProductHistorical) rowFirst
ORDER BY 1
数据
IdProductHistorical IdProduct DateChange Col1 Col2 Col3
------------------- ----------- ----------------------- ----------- ----------- -----------
1 1 2013-01-01 00:00:00.000 1 2 3
2 1 2013-01-02 00:00:00.000 1 2 3
3 1 2013-01-03 00:00:00.000 1 2 3
4 1 2013-01-04 00:00:00.000 1 1 3
5 1 2013-01-05 00:00:00.000 1 1 3
6 1 2013-01-06 00:00:00.000 2 2 2
7 1 2013-01-07 00:00:00.000 2 2 2
8 1 2013-01-08 00:00:00.000 2 2 2
9 1 2013-01-09 00:00:00.000 1 2 3
10 2 2013-01-10 00:00:00.000 1 1 1
11 2 2013-01-11 00:00:00.000 1 1 2
结果
DateChange Col1 Col2 Col3
----------------------- ----------- ----------- -----------
2013-01-01 00:00:00.000 1 2 3
2013-01-04 00:00:00.000 1 1 3
2013-01-06 00:00:00.000 2 2 2
2013-01-09 00:00:00.000 1 2 3
你如何对待Linq？
第一种方法
var query=(
from ph in ProductHistorical.Where(p=>p.IdProduct==1)
orderby ph.DateChange ascending
select new ProductHistoricalItem
{
DateChange = ph.DataChange,
Col1 = ph.Col1,
Col2 = ph.Col2,
Col3 = ph.Col3
});
List<ProductHistoricalItem> listResult=new List<ProductHistoricalItem>();
ProductHistoricalItem previous = null;
foreach (ProductHistoricalItem item in query)
{
if (previous == null ||
previous.Col1 != item.Col1 ||
previous.Col2 != item.Col2 ||
previous.Col3 != item.Col3)
{
listResult.Add(item);
previous = item;
}
}
效率不高。如何在不使用循环的情况下完成它？
答案 0 :(得分：2)
基本上，我试图准确应用你的逻辑并将其转换为Linq代码。
var linqQuery = context.ProductHistoricals
.SelectMany(ph => context.ProductHistoricals, (ph, phPost) => new { ph = ph, phPost = phPost }) // cross join
.Where(a => a.ph.IdProduct == a.phPost.IdProduct
&& a.ph.IdProductHistorical > a.phPost.IdProductHistorical
&& (
a.phPost.Col1 != a.ph.Col1
|| a.phPost.Col2 != a.ph.Col2
|| a.phPost.Col3 != a.ph.Col3))
.Select(a => a.ph)
.GroupBy(p => new { p.IdProduct, p.Col1, p.Col2, p.Col3 })
.Select(p => p.OrderBy(phPost => phPost.IdProductHistorical).FirstOrDefault())
.Union
(
// add first row
context.ProductHistoricals
.GroupBy(t => t.IdProduct)
.Select(t => t.OrderBy(p => p.IdProductHistorical).FirstOrDefault())
);
此查询返回
1 2013-01-01 1 2 3
1 2013-01-04 1 1 3
1 2013-01-06 2 2 2
1 2013-01-09 1 2 3
2 2013-01-10 1 1 1
2 2013-01-11 1 1 2
供参考，这是生成的SQL：
SELECT [t10].[test], [t10].[IdProductHistorical], [t10].[IdProduct], [t10].[DateChange], [t10].[Col1], [t10].[Col2], [t10].[Col3]
FROM (
SELECT [t5].[test], [t5].[IdProductHistorical], [t5].[IdProduct], [t5].[DateChange], [t5].[Col1], [t5].[Col2], [t5].[Col3]
FROM (
SELECT [t0].[IdProduct], [t0].[Col1], [t0].[Col2], [t0].[Col3]
FROM [dbo].[ProductHistorical] AS [t0], [dbo].[ProductHistorical] AS [t1]
WHERE ([t0].[IdProduct] = [t1].[IdProduct]) AND ([t0].[IdProductHistorical] > [t1].[IdProductHistorical]) AND (([t1].[Col1] <> [t0].[Col1]) OR ([t1].[Col2] <> [t0].[Col2]) OR ([t1].[Col3] <> [t0].[Col3]))
GROUP BY [t0].[IdProduct], [t0].[Col1], [t0].[Col2], [t0].[Col3]
) AS [t2]
OUTER APPLY (
SELECT TOP (1) 1 AS [test], [t3].[IdProductHistorical], [t3].[IdProduct], [t3].[DateChange], [t3].[Col1], [t3].[Col2], [t3].[Col3]
FROM [dbo].[ProductHistorical] AS [t3], [dbo].[ProductHistorical] AS [t4]
WHERE ([t2].[IdProduct] = [t3].[IdProduct]) AND ([t2].[Col1] = [t3].[Col1]) AND ([t2].[Col2] = [t3].[Col2]) AND ([t2].[Col3] = [t3].[Col3]) AND ([t3].[IdProduct] = [t4].[IdProduct]) AND ([t3].[IdProductHistorical] > [t4].[IdProductHistorical]) AND (([t4].[Col1] <> [t3].[Col1]) OR ([t4].[Col2] <> [t3].[Col2]) OR ([t4].[Col3] <> [t3].[Col3]))
ORDER BY [t3].[IdProductHistorical]
) AS [t5]
UNION
SELECT [t9].[test], [t9].[IdProductHistorical], [t9].[IdProduct], [t9].[DateChange], [t9].[Col1], [t9].[Col2], [t9].[Col3]
FROM (
SELECT [t6].[IdProduct]
FROM [dbo].[ProductHistorical] AS [t6]
GROUP BY [t6].[IdProduct]
) AS [t7]
OUTER APPLY (
SELECT TOP (1) 1 AS [test], [t8].[IdProductHistorical], [t8].[IdProduct], [t8].[DateChange], [t8].[Col1], [t8].[Col2], [t8].[Col3]
FROM [dbo].[ProductHistorical] AS [t8]
WHERE [t7].[IdProduct] = [t8].[IdProduct]
ORDER BY [t8].[IdProductHistorical]
) AS [t9]
) AS [t10]
答案 1 :(得分：2)
如果您愿意实现自己的LINQ扩展方法，可以使用“WhereWithPrevious”版本（原始实现是SelectWithPrevious，来自here），如下所示：
public static IEnumerable<TSource> WhereWithPrevious<TSource>
(this IEnumerable<TSource> source,
Func<TSource, TSource, bool> selector)
{
using (var iterator = source.GetEnumerator())
{
if (!iterator.MoveNext())
{
yield break;
}
TSource previous = default(TSource);
// return the first item always
yield return iterator.Current;
while (iterator.MoveNext())
{
if(previous != null && selector(previous, iterator.Current))
{
yield return iterator.Current;
}
previous = iterator.Current;
}
}
}
，查询将如下所示：
ProductHistorical.Where(p => p.Id == 1)
.Select(
p => new
{
DateChange = p.Date,
Col1 = p.Col1,
Col2 = p.Col2,
Col3 = p.Col3
})
.OrderBy(p => p.DateChange)
.WhereWithPrevious((prev, curr) =>
prev.Col1 != curr.Col1
|| prev.Col2 != curr.Col2
|| prev.Col3 != curr.Col3)
答案 2 :(得分：0)
怎么样？
int? col1 = null;
int? col2 = null;
int? col3 = null;
ProductHistorical.GroupBy(p=>p.IdProduct)
.Select(grp => grp.OrderBy(p=> p.DateChange)
.Select(p => {
var changed = col1 != p.Col1 || col2 != p.Col2 || col3 != p.Col3;
col1 = p.Col1; col2 = p.Col2; col3 = P.Col3;
return new { p = p, changed = changed };
}
.Where(p => p.changed)
.Select(p => p.p)
)
.SelectMany(p => p)