I keep tables on different .sdf files because it's easy to manage them, ie; back up only changed db file, etc, plus in future db size might bigger and there is -4GB limit-
I need to join the tables and this will be my first -possibly LINQ- attempt. I know there are tons of examples/documents but a simple example would be nice to start.
This is the query for MS SQL Server:
SELECT personID, personPin, personName, seenTime
FROM db1.personList
LEFT JOIN db2.personAttendances on personID = seenPersonID
ORDER BY seenTime DESC
I think LINQ will be the way to go as you're querying across 2 different contexts. LINQ joins are quite easy: http://msdn.microsoft.com/en-gb/vcsharp/ee908647
Something like:
var q = from c in db1Context.personList
join p in db2Context.personAttendances on c.personID equals p.seenPersonID
select new { Category = c, p.ProductName };
I don't think SqlCE supports linking at the Db (SQL) level.
That means you'll have to use Linq-to-Objects. The example query has no WHERE clause so you can simply load the entire tables into Lists. But when the datasets get bigger that may not be acceptable.
Related
I was practicing today when I realized that there are two ways linq to sql can retrieve data from db, I created two datagrid and used the two different ways to populate each of these datagrids and they produced the same result.
The first method is using joins to get data from related tables, and the other methods is using linq query like an object to access related tables. The code is shown below:
NorthWindDataContext dbContext = new NorthWindDataContext();
var orders = from ord in dbContext.Orders
select new { ord.ShipCountry , ord.Customer.ContactName};
var orders2 = from ord in dbContext.Orders
join cust in dbContext.Customers on ord.CustomerID equals cust.CustomerID
select new
{
ord.ShipCountry, cust.ContactName
};
var data = orders2;
DataGrid.ItemsSource= orders;
DataGrid2.ItemsSource = orders2;
My question like the title is if it is entirely necessary to use joins, because I find them really cumbersome to use sometimes.
You need to use something that gets you from the order to the customer.
Join can do this. This is how the second query works.
Having the order "know" about the customer can do this. This is how the first query works.
If your data provider is aware of the connection between order and customer then these will amount to the same thing.
If your data provider is not aware of the connection, then the approach in the first example would result in N + 1 look ups instead of 1.
A linq-friendly ORM will generally be aware of these connections as long as the appropriate relationship-marking attributes are present (just what that is differs between Linq2SQL, EF, NHibernate, etc.).
It's still important to know the join approach for cases where either the relationship isn't known about by the provider, or you have a reason to join on something other than a foreign-key relationship.
The answer is "sort of". Since you're using an ORM such as Linq-to-Sql, no you don't directly need to call join within your linq queries to accomplish what you're trying to do.
However, when the ORM activates the query it will generate actual SQL code that'll have a join statement in it to get the results you're querying. Since you're using an ORM though, the data returned is mapped to objects, and since Customer has a relationship between the objects, the relationship will also be translated to from the database INTO the objects.
ord.Customer.ContactName
The above statement is most likely translated to a JOIN statement performing an INNER JOIN between Customer & Orders.
Due to this, both of your LINQ queries most likely generating similar SQL queries. Both of which has a JOIN statement in them. Because the relationships between your objects also exists within the database (and everything is mapped together showing this relationship) you don't directly need to use join within a LINQ statement.
I would like to know how to efficiently count (SQL server side) the amount of distinct count of results for a specific range of a related entity that has a many to many relationship.
This is the current situation in entity Framework:
Table1 1<------->∞ Table2
Table2 ∞<------->∞ Table4
Table 2 and Table 4 have a many to many relationship and are linked with Table3 in SQL.
What I want is the distinct count of table4 results related to a specific range of Table1.
In LinQ to SQL the query is this:
(from dc in Table1
join vc in Table2 on dc.Table1Id equals vc.Table2Id
join vcac in Table3 on vc.Table2Id equals vcac.Table3Id
join ac in Table4 on vcac.Table3Id equals ac.Table4Id
where dc.Table1Id > 200000
group ac by ac.Table4Id into nieuw
select new { acid= nieuw.Key}).Count()
This lets SQL server return the count directly.
Because the extra table for the many to many relation ( Table3) is gone, I have had problems converting this query to L2E in query syntax. ( since I cannot join table 4 with table 2 with an inner join).
I have this in chained syntax, however, is this efficient ( does this fetch the whole list, or does it let SQLserver do the count, as I'm not sure this is an efficient way to select, Table 2 contains about 30.000 entries, I don't want it to fetch this result just to count it):
context.Table4.Where(a => a.Table2.Any(v => v.Table1Id > 200000)).Select(a => aTable4Id).Distinct().Count();
How would I go converting the Linq2SQL query into L2E in the query syntax ? Or is the chained syntax fine in this situation ?
The .Select() method uses deferred execution, meaning it won't actually run the query until you need the results. At that point in the chain it still exists only as a query definition. Then you modify with .Distinct() before getting .Count() - which does query the database using a SQL GROUP BY statement. So you should be good.
I have a set of (not very well normalised or relational) tables named
PLAN,
GROUP,
PRODUCT
CLIENT
Most have linkage i.e.
PLAN -> CLIENT on clno
GROUP to PRODUCT on PRODCD
However, the linkage between PLAN and GROUP is tricky. A plan has 2 field of interest GRPNO and PRODCD.
What I want to do is if GRPNO != 0 then join GROUP on GRPNO. However if GRPNO = 0 then I want to join GROUP on PRODCD.
The frustrating thing is that the fileds I want to return in my queries are the same across the board I just need to be able to vary the join, or join the same table twice.
The best I can come up with is 2 queries and merge them using datasets, or possibly using a union.
Is there a nifty way to do this in one select?
I should point out I am access Foxpro over ODBC to do this.
Thank you!
You can do:
JOIN GROUP AS G ON
(PL.GRPNO = 0 AND G.PRODCD = PL.PRODCD) OR
(PL.GRPNO !=0 AND G.GRPNO = PL.GRPNO)
However it would surprise me if this is faster than using UNION ALL.
I have a task to construct a dynamic query (or algorithm) based on existing query with user choosed fields. Let me explain:
Lets say I have a function
ConstructQuery(string inputQuery, string[] mandatoryTables, string[] userFields) with 2 input parameters:
inputQuery: string query with many fields and tables, joins and where conditions
mandatoryTables: a list of mandatory tables
userFields: a list of fields that user choose in some web or desktop app
Function would have to return optimized query with tables and joins that are only needed for query to succeed.
inputQuery is for example constructed like this:
SELECT
Table1.SomeFieldA,
Table2.SomeFieldB,
Table2.SomeFieldC,
Table3.SomeFieldD
FROM Table1
JOIN Table2 ON Table1.Code = Table2.Code
JOIN Table3 ON Table2.Code = Table3.Code
WHERE Table1.SomeConditionField = "xyz"
userFields are: SomeFieldB, SomeFieldC
mandatoryTables: Table1
So the expected query is:
SELECT
Table2.SomeFieldB,
Table2.SomeFieldC
FROM Table1
JOIN Table2 ON Table1.Code = Table2.Code
WHERE Table1.SomeConditionField = "xyz"
My question is: is there a tool of some sort for solving this kind of problems or how you guys would solve it? I'm thinking of binary trees…
Regards,
Jani
This is something called join removal. This is (very) hard. Just parsing the query is nontrivial, then You'd have to analyze semantics, consider what are the unique keys, what are foreeign keys to have a chance to remove some tables. In Your example: the algorithm would have to know that table3.code is unique, and a foreign key to table2.code, otherwise the queries are not equivalent.
It could be easier to generate the right query in the first place. This is what some ORMs do.
I am just wondering about how we can determine whether to use join or not in linq to sql.
Eg. let say if we have two tables like this
Table 1 Customer
id
name
Table 2 addresstype
id
address1
customerid
and
var address = from cu in Customer
from ad in addresstype
where cu.id == ad.customerid
select ad;
or
var address = from cu in Customer
join ad in addresstype on cu.id equals ad.customerid
select de;
Is both way are the same. Is there any difference in performance?
Also the second method, will it come up with an error if there isn’t any matching?
Are you using linq to entities or linq to SQL? If its the former then you can avoid both of these by defining your relationships in the model and using navigation properties. This would be the clearest way of doing things
Basically, these two LINQ queries are equivalent to the following SQL queries:
select ad.*
from Customer cu, AddressType ad
where cu.ID == ad.CustomerID -- I assume this was meant by the OP
and
select ad.*
from Customer cu
inner join AddressType ad on cu.id = ad.CustomerID;
The difference between these two queries is mostly semantic, since the database will do the same thing in both cases and return a same result set for both queries.
I would prefer the join syntax in both SQL and LINQ since it defines an explicit relationship between the two tables/entities, that is only implied in the join-less version.
These are seems same query, they return same result but I don't know which one can be a faster, it should be bench marked.
But, In the case of linq2sql I prefer correlated subquery over join, because currently if you want t check the equation two element you should use syntax of:
new {X,Y} equals new {X',Y'}
in join and if you have more than this equations you should convert it to nested query. So I Prefer to have a more readable code which uses minimum differences in difference actions.
To throw a third and more prefered method into the mix with LINQ to SQL, use associations between the tables (even if you don't have them set up in your database). With that in place, you can navigate the object graph rather than using joins:
var query = from cu in Customer
from ad in cu.Addresses
select ad;
Note: when querying the object graphs, LINQ to SQL translates the join into a left outer join where-as the join/where syntax by default is an inner join.
Joins in LINQ should be used when there isn't a natural relationship between the objects. For example, use a join if you want to see the the listing of stores that are in the same city as your customers. (Join Customer.Address.City with Store.Address.City).
There should not be a difference between these two queries. I actually wondered this question myself a few months ago. I verified this through LINQPad. It's a free tool that you can download and actually see the generated SQL of any LINQ query (this is the query that is sent to the database).
The generated SQL should be the same for these two queries.
If you're doing this through Visual Studio, there is also a way you can see the generated SQL as well.