Is there's way to execute SQL queries directly on dataset or datatable . Query which need to be executed is much complex and it may be difficult to implement with Linq .
Is there any similar way to query directly from dataset or datatable ?
Is there any tools available to convert complex queries to linq ?
There is a DataTable.Select() method, which returns rows that match certain criteria.
The syntax is a little limited, however. Example:
DataTable.Select("Date > #1/1/2014# AND Name = \"John\"");
Should select any rows where the Date column has a date value that's 1/1/2014 or later, and the Name column is "John". Also handy is using String.Format() with it.
DataTable.Select(String.Format("Date > {0} AND Name = {1}", date, name));
This is severely limited compared to fixing it in the actual query, though.
try to use tools like LinqPad or some thing similiar
Download LinqPad
also one other way is to create your complex queries in database as VIEW and then write a simple select from VIEW!
another approach is, using MethodChain
What is the correct way to chain methods in .Net
Related
I use the PreprocessQuery method to extend a query in lightswitch.
Something like this:
query = (from item in query
where (validIDs.Contains(item.tableIDs.myID)) &&
elementCount[item.ID] <= maxEleCount)
select item);
Where validIDs is a HashSet(int) and elementCount is a Dictionary(int, int).
the first where clause is working fine, but the second -> elementCount[item.ID] <= maxEleCount
is not working.
What i want to do is to filter a table by some IDs (validIDs) and check also if in another table the number of entries for every of this IDs does not exceed a limit.
Any ideas?
EDIT
I found a solution. Instead of a Dictionary I also used a HashSet for the second where clause. It seems it is not possible to do the Dictionary lookup inside the LINQ statement for some reason (?)
First, although being a bit pedantic, what you're doing in a PreProcessQuery method is "restricting" records in the query, not "extending" the query.
What you put in a LING query has to be able to be processed by the Entity Framework data provider (in the case of LS, the SQL Server Data Provider).
Sometimes you'll find that while your LINQ query compiles, it fails at runtime. This is because the data provider is unable to express it to the data store (again in this case SQL Server).
You're normally restricted to "primitive" values, so if you hadn't said that using a Dictionary actually worked, I would have said that it wouldn't.
Any time you have a static (as in non-changing) value, I'd suggest that you create a variable outside of your LINQ query, then use the variable in the LINQ query. By doing this, you're simply passing a value, the data provider doesn't have to try to figure out how to pass it to the data store.
Reading your code again, this might not be what you're doing, but hopefully this explanation will still be helpful.
I have a query that returns a resultset. And I want to apply filter and sorting on the resultset.
Can someone help me understand if I use the query in LINQ (I'm using EF 4.0), will I be able to get deferred executuin so that when i apply filter/sort in entity model, the execution happens only one time (deffered)
Thanks in advance!
Regards,
Bhavik
If the query takes no parameters, then yes, as you could make a view that calls that sproc, expose the view in your model, then query it.
If it takes parameters, then if you need the sort/filter done server-side, then I think you'd have to add a wrapper sproc (or modify the existing one) to pass in the sort and filter to perform (basically, do it manually, but at least server-side).
Alternatively, you could write the sql to do it server side (sproc results into temp table, then select from that temp table and apply filtering, still manually) and then ExecuteStoreQuery
No, you can't defer the execution of a linq filtering to the stored proc in sql. The stored proc will be executed first, a resultset will be returned, you can then cast it to a list of your object types, once done that you can filter using Linq.
You can easily cast the resultset to a list of your objects using context.Translate<>
Have a look to these links :
enter link description here
List item
Of course the query (in your code) will not be evaluated until you cast it to a list, so you can concatenate all the filtering you want to your resultset and then call the ToList() to get the results.
I have an xml file which contains query details like
<queries>
<query>
<name>GetStudentById</name>
<statement>select student_name from student where student_id=#id</statement>
</query>
</queries>
In C# code, the user will call a method ExecuteQuery(string queryname, hashtable params)
I will retrieve the query statement from the xml file using the query name.
The params hash table will contain values like
params['#id']=5
I will then use ADO .NET to construct the query and execute it at runtime using ExecuteReader after constructing the query statement and passing it the parameter list.
I want to know if it is possible to do it by using LinQ. Can someone show me how to execute the statement as a LinQ query?
Thanks,
David
Have a look at this SO post. Look at the answer from john skeet where he explains how to use the CSharpCodeProvider to execute the statement at runtime.
You will however have to convert your statement from sql to linq yourself. If there is no particular reason for it having to be linq, I would suggest keeping it in sql because at the end of the day you are going to parse sql to linq and then back to sql.
Even with dynamic linq you will have a problem because you will always have to specify the from class in your query.
Another option is that if you want to use this with your entity framework model, it would be way easier to use entity sql, have a look here and here for more info.
It's a considerable leap to go from using string query statements via ADO.NET to Linq to SQL. In the first instance you need to build your Linq to SQL entity model/DataContext. How you go about parsing a sql statement from text to Linq I'm not entirely sure, but if I wanted to look into how to achieve this I would be reading up on expression trees with a view to building a parser that takes your string query and referers to your entity model in order to produce a linq query.
Found something called LinqTextQueryBuilder at codeplex and another MS Library.
Links below
LinqTextQueryBuilder
http://msdn.microsoft.com/en-us/vstudio/bb894665.aspx
How can I return first 100 records using Linq?
I have a table with 40million records.
This code works, but it's slow, because will return all values before filter:
var values = (from e in dataContext.table_sample
where e.x == 1
select e)
.Take(100);
Is there a way to return filtered? Like T-SQL TOP clause?
No, that doesn't return all the values before filtering. The Take(100) will end up being part of the SQL sent up - quite possibly using TOP.
Of course, it makes more sense to do that when you've specified an orderby clause.
LINQ doesn't execute the query when it reaches the end of your query expression. It only sends up any SQL when either you call an aggregation operator (e.g. Count or Any) or you start iterating through the results. Even calling Take doesn't actually execute the query - you might want to put more filtering on it afterwards, for instance, which could end up being part of the query.
When you start iterating over the results (typically with foreach) - that's when the SQL will actually be sent to the database.
(I think your where clause is a bit broken, by the way. If you've got problems with your real code it would help to see code as close to reality as possible.)
I don't think you are right about it returning all records before taking the top 100. I think Linq decides what the SQL string is going to be at the time the query is executed (aka Lazy Loading), and your database server will optimize it out.
Have you compared standard SQL query with your linq query? Which one is faster and how significant is the difference?
I do agree with above comments that your linq query is generally correct, but...
in your 'where' clause should probably be x==1 not x=1 (comparison instead of assignment)
'select e' will return all columns where you probably need only some of them - be more precise with select clause (type only required columns); 'select *' is a vaste of resources
make sure your database is well indexed and try to make use of indexed data
Anyway, 40milions records database is quite huge - do you need all that data all the time? Maybe some kind of partitioning can reduce it to the most commonly used records.
I agree with Jon Skeet, but just wanted to add:
The generated SQL will use TOP to implement Take().
If you're able to run SQL-Profiler and step through your code in debug mode, you will be able to see exactly what SQL is generated and when it gets executed. If you find the time to do this, you will learn a lot about what happens underneath.
There is also a DataContext.Log property that you can assign a TextWriter to view the SQL generated, for example:
dbContext.Log = Console.Out;
Another option is to experiment with LINQPad. LINQPad allows you to connect to your datasource and easily try different LINQ expressions. In the results panel, you can switch to see the SQL generated the LINQ expression.
I'm going to go out on a limb and guess that you don't have an index on the column used in your where clause. If that's the case then it's undoubtedly doing a table scan when the query is materialized and that's why it's taking so long.
I have a ASP.Net page using ADO to query MS access database and as a learning exercise i would like to incorporate LINQ. I have one simple table called Quotes.
The fields are: QuoteID, QuoteDescription, QuoteAuthor, QuoteDate. I would like to run simple queries like, "Give me all quotes after 1995".
How would i incorporate LINQ into this ASP.Net site (C#)
Basically, my question is does LINQ work for MS Access ??
LINQ to SQL doesn't support Access (that is, there's no Access/Jet provider for LINQ), but you can query a DataSet with LINQ. This means that you fill your DataSet with any possible data from your database that you might need in your results, and then you filter on the client side. After you have a typed DataSet, and you Fill() it with a TableAdapter, you do something like this:
var year = 1995; // you can pass the year into a method so you can filter on any year
var results = from row in dsQuotes
where row.QuoteDate > year
select row;
You'll have to decide whether this is worth it. You'd have to fill your DataSet with all the quotes, then use LINQ to filter on just those quotes that are after 1995. For a small amount of data, sure, why not? But for a very large amount of data, you'll need to make sure it won't be too slow.
If you're using a DataSet, though, you can write custom queries that become new TableAdapter methods. So you can put the correct SQL for your query in a FillByYear() method in your TableAdapter and use that to fill your typed DataTable. That way you're only getting back the data you need.
If you go this route, remember that Access/Jet uses positional parameters, not named parameters. So instead of
SELECT * FROM Quotes WHERE Year(QuoteDate) > #Year
you'd use something like this:
SELECT * FROM Quotes WHERE Year(QuoteDate) > ?
I don't think LINQ to SQL supports Access. However, if your table is sufficiently small to fit into memory, LINQ to DataSet will let you query datatables etc pretty easily - especially strongly typed datasets.