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Combine two datatable with common column c#

I have two table as below
Table1
IntervalID(pK)
1
2
3
Table 2
IntervalID Name
1 XXX
Output should be (Table1+Table2)
IntervalID Name
1 XXX
2 NULL
3 NULL
I have done like below
Table1.Merge(Table2);
But it does not give desired output

You could achieve this by using Left join, like the following code:
var result = (from t1 in table1
join t2 in table2 on t1.IntervalId equals t2.IntervalId into leftedResults
from leftedResult in leftedResults.DefaultIfEmpty()
select new { t1.IntervalId, Name = leftedResult?.Name }).ToList();
Note that, you can't merge two DataTables that not have similar schemas.
Left join for DataTables :
var result = (from t1 in dataTable1.AsEnumerable()
join t2 in dataTable2.AsEnumerable() on t1.Field<int>("IntervalId") equals t2.Field<int>("IntervalId") into leftedResults
from leftedReult in leftedResults.DefaultIfEmpty()
select new { IntervalId = t1.Field<int>("IntervalId"), Name = leftedReult?.Field<string>("Name") }).ToList();
I hope you find this helpful.

All combinations - LINQ query

I have 3 tables(Objects in my OR mapping model, whatever). Let's call them TABLE1, TABLE2, and TABLE3 => actual names are in Czech, i think this will be better as example.
1)TABLE1 has 1-N relationship with TABLE2.
2)TABLE1 has also 1-N relationship with TABLE3. So the tables look something like this:
TABLE2
{
int ID;
nvarchar attributeT2;
int TABLE1_FK;
}
TABLE1
{
int ID;
}
TABLE3
{
int ID;
nvarchar attributeT3
int TABLE1_FK;
}
Now i need all combinations of: TABLE2 records where attributeT2 == "T2" and TABLE3 records where attributeT3 == "T3" BUT those two must have the same TABLE1_FK(= they must be joined by TABLE1 record).
So if there were 2 records in TABLE2 with TABLE1_FK ==1, and 2 records in TABLE3 with TABLE1_FK == 1, i need all combinations == 4 pairs. I could do this by foreach cycle, and querying the DB in each step, but i think that would be very ineffective, and i was wondering if there was a better way.
I thought it could be something like this:
var query= from x in context.TABLE2
where x.attributeT2 == "T2"
join y in context.TABLE3 on x.TABLE1_FK == y.TABLE1_FK
where y.attributeT3 == "T3"
select new WrapperClass(x,y);
But the combinations are wrong - the pair does not always have the same TABLE1_FK. Thanks a milion for any response.

I didn't check it myself, but maybe worths to try. The idea is to join on FK columns in TABLE2 and TABLE3 and have where condition for attibuteT3 and attributeT2
var query= from x in context.TABLE2
join y in context.TABLE3 on x.TABLE1_FK == y.TABLE1_FK
where y.attributeT3 == "T3" && x.attributeT2 == "T2"
select new WrapperClass(x,y);

I am SO sorry, but 2 minutes after i'd hit the submit button, i thought of something i haven't tried.
var asdf = from x in context.TABLE1
join y in context.TABLE2 on x.ID equals y.TABLE1_FK
where y.attributeT2 == "T2"
join z in context.TABLE3S on x.ID equals z.TABLE1_FK
where z.attributeT3 == "T3"
select new WrapperClass(y,z);
It worked. Thanks a lot anyway Michael.

Select from multiple tables using linq

I am trying to execute the following SQL statement using linq:
SELECT TTT.SomeField
FROM Table1 as T, Table2 as TT, Table3 as TTT
WHERE (T.NumberID = 100 OR T.NumberID = 101)
AND T.QuestionID = 200
AND T.ResponseID = TT.ResponseID
AND TT.AnswerID = TTT.AnswerID
Essentially getting one field from a third table based on primary/foreign key relationships to the other 2 tables. It is expected to have a single result every time.

var query = from t in db.Table1
join tt in db.Table2 on t.ResponseID equals tt.ResponseID
join ttt in db.Table3 on tt.AnswerID equals ttt.AnswerID
where (t.NumberID == 100 || t.NumberID == 101) && t.QuestionID == 200
select ttt.SomeField
If you always expect single result, you can wrap this in ().Single(), or, if there might be no results found, in ().SingleOrDefault().

If I understand you correct. You should read something about Joins I guess.
here

LINQ equivalent for SQL

I am trying to convert a ASP.NET project to Entity framework. How to re-write the following query to its LINQ equivalent?
SELECT {Table1 objects}
FROM [Table1] tb1
INNER JOIN [Table2] tb2
ON tb1.Table1ID = tb2.fk_Table1ID
WHERE tb2.fk_attrib1 = '123' AND tb2.fk_attrb2 = '345'
ORDER BY tb1.attrib1
The result is a collection of Table1 objects.
Here Table1 and Table2 correspond to object System.Data.Objects.ObjectSet of ADO.NET Entity Framework.

var results = from tb1 in Context.Table1
join tb2 in Context.Table2 on tb1.Table1ID == tb2.fk_Table1ID
where tb2.fk_attrib1 == "123" && tb2.fk_attrb2 == "345"
orderby tb1.attrib1
select tb1;

Something like this:
context.Table1
.Where( o => o.Table2s.Any( o2 =>
o2.fk_attrib1 == '123' &&
o2.fk_attrib2 == '345' ) )
.OrderBy( o => o.attrib1 )
.ToList();
BTW, LINQPad is great for trying out L2E queries.

This should help you a little bit. I suppose the main problem is with JOIN clause - in EF you can use NavigationProperties and don't need to worry about joining tables - EF will take care of that for you.
Also you are trying to filter on column from joined table. This you can do using Any method to find all Table1 elements that are connected to Table2 where those referenced elements have certain properties/columns. You should also get familiar with All method, as it might be useful to you in future.
from t1 in context.Table1
where t1.Table2s.Any(t2.fk_attrib1 == "123" && t2 => t2.fk_attrb2 == "345")
order by t1.attrib1
select t1;
Edit:
I assume that there is 1:n relationship between Table1 and Table2 which results in enumerable collection as NavigationProperty in Table1 objects.
Edit2:
Fixed error in code - didn't noticed that both attributes are from Table2 not Table1

Should be something like this:
var result = (from tb1 in Table1
from tb2 in Table2
where tb1.Key == tb2.Key &&
tb2.fk_attrib1 = '123' &&
tb2.fk_attrb2 = '345'
select ione).OrderBy(p=>p.attrib1);
Hope this helps.

Need help with an Opposite to Inner join Query using LINQ

I have two tables in an XML Dataset. T1, T2. Each of the tables has a ID column.
T1 has a list of Customers
T2 has a list of Orders
I want to build a LINQ query that returns only the ID of the customers that do not have orders. In other words customer ID's that do not exist in the T2 table.
Oh yea, I'm using C#
Thanks!

This requires an outer join and a check on null.
var result = from c in Customers
join d in Details on d.CustomerID equals c.ID into g
where !g.Any()
select c;

I think this will work (please adapt to your DataSets):
var query = from c in T1
where !(from o in T2 select o.CustomerID)
.Contains(c.CustomerID)
select c;

You just need to us a where clause and all:
T1.Where( item1 => T2.All( item2 => item1.ID != item2.ID ) );