I'm trying to execute a left join between 2 datatables that will return all records from the left table without a corresponding value in the right table on the join criteria. As of now I have the following which returns nothing:
Dim Query1 = From exasset In dtExistingAssets _
GroupJoin asset In dtNewAssets _
On exasset("ACCOUNT_NAME") Equals asset("ACCOUNT_NAME") _
Into results = Group _
From f In results.DefaultIfEmpty _
Where IsDBNull(f) _
SelectNewWith _
{ //...
I've seen several references to using Any but I wasn't able get the syntax correct. Can anyone please help out? This is something that is really simple to accomplish in SQL but seems a lot more complicated in LINQ.
I think the problem is the IsDBNull(f), a left join will result in a null value (Nothing in VB) not a DBNull value. I think you should change it to: ``
...
From f In results.DefaultIfEmpty _
Where f is Nothing
I would use the strongly typed DataRow extension methods like Field which also support nullables.
Dim query = From exAsset In dtExistingAssets
Group Join newAsset In dtNewAssets
On exAsset.Field(Of String)("ACCOUNT_NAME") Equals newAsset.Field(Of String)("ACCOUNT_NAME") Into Group
From joinedAssets In Group.DefaultIfEmpty()
Where joinedAssets.Field(Of String)("ACCOUNT_NAME") Is Nothing
If you just want to know the new accounts, you can also use the efficient Enumerable.Except:
Dim existingAccounts = From exRow In dtExistingAssets
Select exRow.Field(Of String)("ACCOUNT_NAME")
Dim newAccounts = From newRow In dtNewAssets
Select newRow.Field(Of String)("ACCOUNT_NAME")
Dim newAccNotInExisting = newAccounts.Except( existingAccounts )
Related
I'm trying to figure Linq out. I'm having very little success and most of the articles are for C# which isn't helping.
I'm trying to make the following work;
Dim query = From r In db.eq_list
Join s In db.interview_main On r.CLIENTCODE Equals s.CLIENTCODE And r.CONTROL Equals s.CONTROL
Select New With {r.UserName, r.CONTROL, r.CLIENTCODE, r.CLIENTLOCATION, r.IDATETIME, r.FIRSTNAME,
r.LASTNAME, If(String.IsNullOrEmpty(s.Code), 0, s.Code)}
dgvOnHold.DataSource = query.ToList
The problem is the IF part. This part can be NULL in the database, but I want it returned as 0 if NULL. I'm putting this into a read-only Datagridview. The original SQL is as follows;
SELECT r.ID,r.UserName,r.CONTROL,r.CLIENTCODE,r.CLIENTLOCATION,r.IDATETIME,r.FIRSTNAME,r.LASTNAME,ISNULL(s.CODE,0) AS CODE
FROM system.eq_list AS r
LEFT JOIN interview.main AS s ON r.CLIENTCODE = s.CLIENTCODE AND r.CONTROL = s.CONTROL;
Can't check this at the moment but how about?
Dim query = From r In db.eq_list
Join s In db.interview_main On r.CLIENTCODE Equals s.CLIENTCODE And r.CONTROL Equals s.CONTROL
Select New With {r.UserName, r.CONTROL, r.CLIENTCODE, r.CLIENTLOCATION, r.IDATETIME, r.FIRSTNAME,
r.LASTNAME, If(s.Code is Nothing, 0, s.Code)}
dgvOnHold.DataSource = query.ToList
I'm trying to query old Access database tables and compare them with SQL Server tables.
They often don't have primary keys, or they have extra fields that had some purpose in the nineties, etc., or the new tables have new fields, etc.
I need to find records - based on a set of fields specified at runtime - that are in one table but not another.
So, I do this kind of query all the time in SQL, when I'm comparing data in different tables:
dim fields_i_care_about as string = "field1, field2, field3"
'This kind of thing gets set by a caller, can be any number of fields, depends on the
'table
dim s as string= ""
dim flds = fields_i_care_about.split(",")
for i as integer = 0 to ubound(flds)
if s > "" then s += " AND "
s += " dysfunctional_database_table." & flds(i) & "=current_database_table." & flds(i)
next
s = "SELECT * from dysfunctional_database_table where not exists (SELECT * from current_database_table WHERE " & s & ")"
====
I'm trying to do this using Linq because it seems like some of the datatype problems with two different database types become less of a headache,
but I'm new to Linq and totally stuck.
I got as far as this:
Put old and new tables into datatables as dt1 and dt2
Dim new_records = _
From new_recs In dt2.AsEnumerable
Where Not ( _
From old_recs In dt1.AsEnumerable Where old_recs(field1) = new_recs(field1) AndAlso old_recs(field2) = new_recs(field2)).Any
Select new_recs
But I can't figure out how to put this part in on the fly -
old_recs(field1) = new_recs(field1) AndAlso old_recs(field2) = new_recs(field2)
So far I've tried:
putting the fields I want to compare and making them a string and just putting that string in as a variable ( I thought I was probably cheating, and I guess I was)
dim str = old_recs(field1) = new_recs(field1) AndAlso old_recs(field2) = new_recs(field2)
From new_recs In dt2.AsEnumerable
Where Not ( _
From old_recs In dt1.AsEnumerable Where str).Any
Select new_recs
It tells me it can't convert a Boolean -
Is there any way to do this without Linq expressions? They seem far more complex than what I'm trying to do here, and they take a lot of code, and also I can't seem to find examples of Expressions where we're comparing two fields in a subquery.
Is there a simpler way? I know I could do the usual EXISTS query using JOIN or IN - in this case I don't need the query to be super fast or anything. And I don't need to use a DataTable or DataSet - I can put the data in some other kind of object.
So I found a lot of sample code that used MethodInfo and reflection and things like that, but I couldn't get any of it to work - these Datarows have a Field method but it requires that you add an (of object) argument before the field name argument and that's tricky to do.
So I'm not sure if this solution is the most efficient way, but at least it works. I'd be interested in finding out whether this way of doing it is efficient and why or why not. It seemed like most people used reflection to do this kind of thing, but I couldn't get that working properly and anyway what I'm trying to do is pretty simple while those methods were pretty complex. I suppose I'm doing Linq with a SQL mindset, but anyway it works.
Dim f As Func(Of DataRow, DataRow, String, Boolean) = Function(d1 As DataRow, d2 As DataRow, s As String)
Dim fields = Split(s, ",")
Dim results As Boolean = True
For k As Integer = 0 To UBound(fields)
Dim obj = DataRowExtensions.Field(Of Object)(d1, fields(k))
Dim obj2 = DataRowExtensions.Field(Of Object)(d2, fields(k))
If obj <> obj2 Then results = False : Exit For
Next
Return results
End Function
Dim new_records = _
From new_recs In dt2.AsEnumerable.AsQueryable()
Where Not ( _
From old_recs In dt1.AsEnumerable.AsQueryable Where f(old_recs, new_recs, id_key)).Any
Select new_recs
Try
Return new_records.CopyToDataTable
Catch ex As Exception
Stop
End Try
I'm completely dense here, but I'm trying to get some stats from a DataTable. One of the columns in the datatable is called "colour".
I need to find out how many of each instance of "colour" are in the datatable.
I'm trying:
Dim q = From p In PGWorkingDataTable _
Group p By p("colour") Into Group _
Select Group
But I get the compiler error at design-time:
"Range variable name can be inferred only from a simple or qualified name with no arguments" on the p("colour") section.
I need some serious guidance here. Thanks for your help.
Joe
You need to specify a name for the group key:
From p In new DataTable() _
Group p By Color = p("colour") Into Group _
Select Group
Writing LINQ in VB.NET is nobody's favorite thing. Try something like the following:
Dim q = From p In PGWorkingDataTable _
Group By colour = p("colour") _
Into colourCount = Count(p("colour")) _
Select colour, colourCount
I hava a strongly typed datatable and a list(of String).
I want to build a linq query to return a datatable of the same type where the fields of a certain column of the table are in the list. I thought of doing a Join, although in normal sql I would have added
SELECT FROM Table WHERE Table.ID IN(...);
This is what I tried in linq.
Dim Families As List(Of String)
Dim Articles As SomeStronglyTypedDataTable
Dim MatchingArticles = From a In Articles.AsEnumerable _
Join f In Families.AsEnumerable On a.FamilyCode Equals f.ToString _
Select New With {}
I'm not sure either if I need to convert the query result back to a datatable nor if that's even possible.
Thanks!
Try the simpler query:
Dim MatchingArticles = From a In Articles.AsEnumerable _
Where Families.Contains(a.FamilyCode)_
Select a
Dim MyMatchingArticlesTable = CopyToDataTable(Of SomeStronglyTypedDataTable) (MatchingArticles)
Yes, you can do this. Instead of Select New ..., select the matching DataRows, Select a, and then use CopyToDataTable(Of T) on the matching rows.
Dim table As DataTable = query.CopyToDataTable()
Dim typedtable As New TypedDataset.TypedDataTable
typedtable.Merge(table)
I was raking my brain trying to get something similar to work, and your code enlightened me.
All I needed was to add the .AsEnumerable() on both sides.
I'm working with C#. Anyway, I think all you need to do is select your table like
Dim MatchingArticles = From a In Articles.AsEnumerable _
Join f In Families.AsEnumerable On a.FamilyCode Equals f.ToString _
Select a;
Well, this is a very old post, but hey, it might help someone else...
If you think this would resolve your question, please mark it as correct, so others will know. You may also want to mark Devart's answer as correct. I tried it and it works.
I need to make a query that look like this is SQL:
SELECT CodProiect, SUM(Valoare)
FROM DET
WHERE CodProiect = 'cod_pr'
GROUP BY CodProiect;
How can I write the same thing in LINQ?
I have attempted this:
dim gac1 = (From ac1 In t_detalii _
Where ac1!CodProiect = cod_pr1 _
Select ac1!Valoare).Sum(Function(ac1) ac1!Valoare)
But it gives me an error "No default member found for type 'Double'."
You need to select ac1 instead of ac1!Valoare (which is the property itself, not the corresponding object)
dim gac1 = (From ac1 In t_detalii _
Where ac1!CodProiect = cod_pr1 _
Select ac1).Sum(Function(ac1) ac1!Valoare)
edit
You can of course group it in the query already:
dim gac1 = (From ac1 In t_detalii _
Where ac1!CodProiect = cod_pr1 _
Group By ac1.CodProiect Into ac2
Select ac2.Sum(Function(ac2) ac2!Valoare)
(something in that direction, I'm not familiar with the vb.net syntax, also I'm writing it without compilersupport atm.