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Nested Select MySQL statements to LINQ

I'm trying to convert the following MySQL statement in to LINQ query format
SELECT * FROM table1 WHERE table1.id IN (SELECT c_id FROM table2 WHERE a_id IN (1, 49) GROUP BY c_id HAVING COUNT(*) = 2) ORDER BY name
Got as far as this, but I'm drawing a blank on how to handle the IN and 2nd SELECT statement
myItems = from c in table1
let id = c.id
where ????
orderby c.name
select c;
Would appreciate some guidance with this please

Try this:
var ids=new[]{1,49};
var innerquery=table2.Where(e=>ids.Contains(e.a_id))
.GroupBy(e=>e.c_id)
.Where(g=>g.Count()==2)
.Select(g=>g.Key);
var myItems = from c in table1
where innerquery.Contains(c.id)
orderby c.name
select c;
First define your inner query,after the group by you will get a collection of IGrouping<TKey, TElement>> that represent a collection of objects that have a common key, filter the groups choosing only those where count==2, and select the keys of those groups. The second part is really easy to understand. I split the process in two queries to do it more readable, but you can merge both query in one.

C# Linq query where Table A column is not equal/doesnt math Table B column join

I have an issue with a c# linq query where I use the != operator, it works well in SQL but when I write the same query in C# it returns a different result, which is the correct way to the the results where table a column doesn't match table b column. Please see my sql query below and then my c# query.
SELECT tba.ID,fa.accountnumber,tba.Account_Number,fa.new_legalname,tba.Legal_Name,fa.new_deliverystatusname, fa.new_deliverystatus,tba.Delivery_Charge
FROM [CRM_Embrace_Integration].[dbo].[CRM_Tarsus_Debtors_Accounts] tba
inner join CRM_MBT_GROUP.dbo.FilteredAccount fa
ON fa.accountnumber collate database_default = tba.Account_Number
where fa.new_legalname collate database_default != tba.Legal_Name
and the Linq query looks like this
var sqlJoinQuery = from accCRM in todaysCRMAccounts
join accSQL in todaysCRMViewAccounts
on accCRM.Account_Number equals accSQL.accountnumber
where accCRM.Legal_Name != accSQL.new_legalname
select new { accCRM.Legal_Name, accSQL.new_legalname };
The SQL query returns the correct result as I want where legal_name(table A) is not equals to legal_name(table B) is the other table.
The Linq query return incorrect result, please assist.

I suggest to try the following Linq as you want the data that aren't in table 2:
var result1 = (from m in db1.Table1
select m).ToList();
var result2 = (from m in db2.Table2
select m).ToList();
var finalResult = (from m in result1
where !(from k in result2
select k.Id).Contains(m.Id)
select m).ToList();
The above will return that aren't in Table2. I hope, this is what you wanted.

Your SQL shows you asking for where they are equal. The LINQ shows you asking for when they are not equal.

Linq return distinct values from table if not exist in another

I am trying to return all distinct rows from Staging below where Staging.CenterCode does not exist in Centers.CenterCode.
At the moment Stagings has around 850 distinct CenterCodes and Centers is empty so I should be getting all of the distinct rows, but count begs to differ :)
Any ideas?
var query =
(from s in db.Stagings
join t in db.Centers on s.CenterCode equals t.CenterCode into tj
from t in tj.DefaultIfEmpty()
where s.CenterCode != t.CenterCode
select s.CenterCode).Distinct();
var c = query.Count();
I only need the unique columns from staging so not sure if I actually need a join with the above as I am not ever using data returned from Centers - I have however tried both and get the same 0 value for count.
Any ideas?

I would not use a join, but use a Contains.
var centerCodesQuery = db.Centers.CenterCode
.Select(x => x.CenterCode);
var query = db.Staging
.Where(x => !centerCodesQuery.Contains(x.CenterCode))
.Select(x => x.CenterCode)
.Distinct();
var c = query.Count();

the join is an inner join. So, if none of the rows in 1 table match the other table on the specified identifier then it will return 0. In yours you are trying to join 1 table with 850 distinct rows with an empty table. This will return 0.
If you actually want to return only those rows in 1 table that aren't in another you can use Except:
var query = (from s in db.Stagings
select s.CenterCode)
.Except(from t in db.Centers
select t.CenterCode);
var c = query.Count();

Looks like you are trying to implement antijoin via left outer join, which is one of the possible ways, but in order to make it work, you need to change
where s.CenterCode != t.CenterCode
to
where t == null

GroupBy query is slow what are the other options?

I need a query to select rows with minimum Insert Date Time. but groupby query is very slow
what can I do instead of groupby?
How can improve the performance of the query?
Is there any other way to write this query using linq?
Number of rows in my table is around 600,000 rows and growing
fullsimnumber isn't indexed but isdeleted and insertdatetime are indexed.
fullsimnumber is a column which consist of three indexed coloumn prec+cod+subscribec.
my problem is with linq queries which always gives timeout exception.
I changed fullsimnumber to groupby prec,cod,subscribec which are indexed but still getting tiemout exception
Im using linq to EF, (code first style) and my query in sql is:
SELECT *
FROM dbo.Sim AS t1
JOIN (SELECT FullSimNumber,MIN(InsertDateTime) AS insd
FROM dbo.Sim
GROUP BY FullSimNumber) AS t2
ON t1.FullSimNumber = t2.FullSimNumber AND t1.InsertDateTime = t2.insd
WHERE t1.IsDeleted = 0
and my query in linq
from s in ADbContext.Numbers
where !s.IsDeleted
group s by s.FullSimNumber
into g
let sd = g.OrderBy(x => x.InsertDateTime).FirstOrDefault()
select sd;

without any data on indexes and amounts of rows and such it is hard to know what exactly is needed to make your query faster. One thing which is worthwhile to try out is this:
SELECT *
FROM dbo.Sim AS t1
CROSS APPLY (
SELECT TOP 1 FullSimNumber
, InsertDateTime AS insd
FROM dbo.Sim t2
WHERE t1.FullSimNumber = t2.FullSimNumber
AND t1.InsertDateTime = t2.insd
ORDER BY InsertDateTime DESC
) AS t2
WHERE t1.IsDeleted = 0
Again this could very well be worse! test it and compare the execution times and load.

How do I join tables with a condition using LLBLGen?

I have the following Sql Query that returns the type of results that I want:
SELECT b.ID, a.Name, b.Col2, b.COl3
FROM Table1 a
LEFT OUTER JOIN Table2 b on b.Col4 = a.ID AND b.Col5 = 'test'
In essence, I want a number of rows equal to Table1 (a) while having the data from Table2 (b) listed or NULL if the condition, 'test', doesn't exist in Table2.
I'm rather new to LLBLGen and have tried a few things and it isn't working. I can get it to work if the condition exists; however, when a requirements change came in and caused me to rewrite the query to that above, I'm at a loss.
Below is the old LLBLGen C# code that worked for existing products but not for the above query:
LookupTable2Collection table2col = new LookupTable2Collection();
RelationCollection relationships = new RelationCollection();
relationships.Add(LookupTable2Entity.Relations.LookupTable1EntityUsingTable1ID, JoinHint.Left);
IPredicateExpression filter = new PredicateExpression();
filter.Add(new FieldCompareValuePredicate(LookupTable2Fields.Col5, ComparisonOperator.Equal, "test"));
table2col.GetMulti(filter, relationships);
Table 1 has 3 records in it. I need the 3 records back even if all items from Table 2 are NULL because the condition doesn't exist. Any ideas?

You've to add your filter to the relation join like this:
relationships.Add(LookupTable2Entity.Relations.LookupTable1EntityUsingTable1ID, JoinHint.Left).CustomFilter = new FieldCompareValuePredicate(LookupTable2Fields.Col5, ComparisonOperator.Equal, "test");