我们正在使用Entity Framework，我们希望执行一个简单的LEFT JOIN。
实际上，这是我们想要在LINQ（可查询）中使用的SQL：
Const strPath As String = "C:\ImportFolder\" 'Directory Path
Dim strFile As String 'Filename
Dim strFileList() As String 'File Array
Dim intFile As Integer 'File Number
'Loop through the folder & build file list
strFile = Dir(strPath & "*.csv")
While strFile <> ""
'add files to the list
intFile = intFile + 1
ReDim Preserve strFileList(1 To intFile)
strFileList(intFile) = strFile
strFile = Dir()
Wend
'see if any files were found
If intFile = 0 Then
MsgBox "No files found"
Exit Sub
End If
'cycle through the list of files & import to Access
'creating a new table called MyTable
For intFile = 1 To UBound(strFileList)
DoCmd.TransferText acImportDelimi, ImportSpec, _
"Raw Data", strPath & strFileList(intFile), -1
'Check out the TransferSpreadsheet options in the Access
'Visual Basic Help file for a full description & list of
'optional settings
Next
MsgBox UBound(strFileList) & " Files were Imported"
我们想出的是：
SELECT
cbl.ID as ClaimBatchLine_ID
,cbl.PurchasePrice
,c.*
,ic.DueDate
,ic.Reference
FROM ClaimBatchLine cbl
INNER JOIN Claim c ON c.ID = cbl.CLaim_ID
LEFT JOIN InvoiceClaim ic ON ic.ID = c.ID
WHERE cbl.ClaimBatch_ID = @claimBatchId
ORDER BY cbl.ID
OFFSET (@recordsPerPage*@page) ROWS
FETCH NEXT @recordsPerPage ROWS ONLY
这会产生以下SQL。
from cbl in ClaimBatchLines where cbl.ClaimBatch_ID == 1
from c in Claims where c.ID == cbl.Claim_ID
from ic in InvoiceClaims.DefaultIfEmpty() where ic.ID == c.ID
select new {cbl, c, ic.Reference}
它生成相同的结果集。这太好了。但是，您可以看到SELECT [t0].[ID],
[t0].[ClaimBatch_ID],
[t0].[Claim_ID],
[t0].[PurchasePrice],
[t1].[ID] AS [ID2],
[t1].[ClaimType_ID],
[t1].[Debtor_ID],
[t1].[CustomerContractRevision_ID],
[t1].[Date],
[t1].[CreatedOn],
[t1].[GrossAmount],
[t1].[OpenAmount],
[t1].[IsProcessedByOpenAmountCalculator],
[t1].[RowVersion],
[t2].[Reference] AS [Reference]
FROM [ClaimBatchLine] AS [t0]
CROSS JOIN [Claim] AS [t1]
LEFT OUTER JOIN [InvoiceClaim] AS [t2] ON 1 = 1
WHERE([t2].[ID] = [t1].[ID])
AND ([t1].[ID] = [t0].[Claim_ID])
AND ([t0].[ClaimBatch_ID] = @p0);
不是我们想要的。我希望它会将其翻译为LEFT OUTER JOIN [InvoiceClaim] AS [t2] ON 1 = 1。
我们做错了吗？或者LINQ to SQL只是次优（关于性能）并且无法理解我们想要的东西。
修改 在LINQPad中，这会产生一些不错的查询
LEFT JOIN InvoiceClaim ic ON ic.ID = c.ID即
from cbl in ClaimBatchLines
join c in Claims on cbl.Claim_ID equals c.ID
join ic in InvoiceClaims on c.ID equals ic.ID into g
from e in g.DefaultIfEmpty()
where cbl.ClaimBatch_ID == 1
select new {cbl, c, e.Reference}
但是在添加这样的分页功能时：
-- Region Parameters
DECLARE @p0 INT= 1;
-- EndRegion
SELECT [t0].[ID],
[Columns left out for brevity]
[t2].[Reference] AS [Reference]
FROM [ClaimBatchLine] AS [t0]
INNER JOIN [Claim] AS [t1] ON [t0].[Claim_ID] = [t1].[ID]
LEFT OUTER JOIN [InvoiceClaim] AS [t2] ON [t1].[ID] = [t2].[ID]
WHERE [t0].[ClaimBatch_ID] = @p0;
它产生了这个'怪物'：
(from cbl in ClaimBatchLines
join c in Claims on cbl.Claim_ID equals c.ID
join ic in InvoiceClaims on c.ID equals ic.ID into g
from e in g.DefaultIfEmpty()
where cbl.ClaimBatch_ID == 1
select new {cbl, c, e.Reference})
.OrderBy(a => a.cbl.ID)
.Skip(0 * 15000)
.Take(15000)
更糟糕的是。当我不通过LINQpad执行相同的LINT-To-EF时，但是在使用EF存储库的代码中，我得到了这个更大的怪物：
-- Region Parameters
DECLARE @p0 INT= 1;
DECLARE @p1 INT= 0;
DECLARE @p2 INT= 15000;
-- EndRegion
SELECT [t4].[ID],
[Columsn left out for brevity...]
FROM
(
SELECT ROW_NUMBER() OVER(ORDER BY [t3].[ID]) AS [ROW_NUMBER],
[Columsn left out for brevity...]
FROM
(
SELECT [t0].[ID],
[Columsn left out for brevity...]
FROM [ClaimBatchLine] AS [t0]
INNER JOIN [Claim] AS [t1] ON [t0].[Claim_ID] = [t1].[ID]
LEFT OUTER JOIN [InvoiceClaim] AS [t2] ON [t1].[ID] = [t2].[ID]
) AS [t3]
WHERE [t3].[ClaimBatch_ID] = @p0
) AS [t4]
WHERE [t4].[ROW_NUMBER] BETWEEN @p1 + 1 AND @p1 + @p2
ORDER BY [t4].[ROW_NUMBER];
这到底是怎么回事！在LINQpad上抓一点看起来很好。但是生产代码中的最终查询非常可怕！ 也许这些查询可以用一些简单的应用程序。在查询更多记录中的500k记录时，这对我来说不太好。我将使用LINQ坚持使用纯SQL表值函数。太遗憾了。
答案 0 :(得分：2)
尝试这种方式：
from cbl in ClaimBatchLines
join c in Claims on c.ID equals cbl.Claim_ID
join ic in InvoiceClaims on ic.ID equals c.ID into g
from e in g.DefaultIfEmpty()
where cbl.ClaimBatch_ID == 1
select new {cbl, c, e.Reference}
有关如何在linq中执行左连接的详细信息，请查看此link
答案 1 :(得分：2)
最简单的方法是将你的位置移到连接处：
query = "acut myeloid leukemia"
document1 = "acut myeloid leukemia normal karyotyp"
document2 = "acut myeloid leukemia"
document3 = "acut normal karyotyp"
Q <- unlist(strsplit(query, " "))
d1 <- unlist(strsplit(document1, " "))
d2 <- unlist(strsplit(document2, " "))
d3 <- unlist(strsplit(document3, " "))
y <- adist(d1,Q)
double_summation1 = 0
for (i in 1:nrow(y-1)) {
for (j in 1:ncol(y-1)) {
double_summation1 = double_summation1 + abs(i-j)
}
}
double_summation1
scatter <- sum(do.call(pmin, lapply(1:nrow(y), function(x)y[x,])))
dist_d_Q1 <- scatter/double_summation1
y <- adist(d2,Q)
double_summation2 = 0
for (i in 1:nrow(y-1)) {
for (j in 1:ncol(y-1)) {
double_summation2 = double_summation2 + abs(i-j)
}
}
double_summation2
scatter <- sum(do.call(pmin, lapply(1:nrow(y), function(x)y[x,])))
dist_d_Q2 <- scatter/double_summation2
y <- adist(d3,Q)
double_summation3 = 0
for (i in 1:nrow(y-1)) {
for (j in 1:ncol(y-1)) {
double_summation3 = double_summation3 + abs(i-j)
}
}
double_summation3
scatter <- sum(do.call(pmin, lapply(1:nrow(y), function(x)y[x,])))
dist_d_Q3 <- scatter/double_summation3
c(dist_d_Q1, dist_d_Q2, dist_d_Q3)
[1] 23
[1] 8
[1] 8
[1] 0.00 0.00 1.75
这将使查询在from cbl in ClaimBatchLines where cbl.ClaimBatch_ID == 1
from c in Claims where c.ID == cbl.Claim_ID
from ic in InvoiceClaims.Where(x => x.ID == c.ID).DefaultIfEmpty()
select new {cbl, c, ic.Reference}
表
left join
答案 2 :(得分：1)
尝试使用linq的join编写它。
var q =
(from cbl in ClaimBatchLines
join c in Claims on cbl.Claim_ID equals c.ID
join tmpIc in InvoiceClaims on c.ID equals tmpIc.ID into g
from ic in g.DefaultIfEmpty()
where cbl.ClaimBatch_ID == 1
select new { cbl, c, ic })
.OrderBy(x => x.cbl.ID)
.Skip(recordsPerPage * page)
.Take(recordsPerPage);