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.
Related
I'm attempting to explicitly join 3 tables using a left outer join in a linq query and am running into linq parsing issues. Performing an inner join parses correctly and returns data but using the left outer fails.
Example:
var query = from p in DatabaseContext.Products
where p.ClientID == clientID
join l in DatabaseContext.Licenses on p.ProductID equals l.ProductID into pl
from pli in pl.DefaultIfEmpty()
join a in DatabaseContext.Articles on p.ArticleID equals a.ArticleID into pa
from pai in pa.DefaultIfEmpty()
select new SomeEntityDTO
{
SomethingFromP = p.Something,
SomethingFromL = pli.Something,
SomethingFromA = pai.Something
};
As both joined tables key off of the first table, I can test each individually by removing the other join, e.g., test the query for p to l and then for p to a. These test queries function perfectly. It's also possible to remove the left outer rule and receive a proper result.
var query = from p in DatabaseContext.Products
where p.ClientID == clientID
join l in DatabaseContext.Licenses on p.ProductID equals l.ProductID
join a in DatabaseContext.Articles on p.ArticleID equals a.ArticleID
select new SomeEntityDTO
... the rest ...
Viewing the offending query in SQL Profiler (top code example) I see that the first two tables are successfully joined, e.g.:
SELECT p.Something, l.Something
FROM Products AS p
LEFT JOIN Licenses AS l ON p.ProductID = l.ProductID
WHERE p.ClientID = 5
ORDER BY p.ProductID
And, then right after this successful query, are another 2 queries (identical to each other):
SELECT a.ArticleID, a.Something, <all fields, even when not specified in query>
FROM Articles AS a
ORDER BY a.ArticleID
The outer joined 3 tables will successfully return an object, as long as I don't attempt to access a field from the "a" table. When doing that, I recieve a Null Exception error, as that table was never really joined.
As stated, removing the outer join rule brings back a successfully joined query.
I have attempted to adjust the linq query figuring that the Linq parser had an issue, but to no avail:
var query = from p in DatabaseContext.Products
from l in DatabaseContext.Licenses.Where(g => g.ProduktID == p.ProduktID).DefaultIfEmpty()
from a in DatabaseContext.Articles.Where(g => g.ArticleID == p.ArticleID).DefaultIfEmpty()
where ....
This parses to a set of CROSS APPLYs that doesn't function at all and the profiled query, when copied into a query editor window, doesn't run at all (as opposed to the 3 individual queries seen in the profiler for the first code example). I have also attempted the more complicated lambdas, which also doesn't work.
Is this an error in the Linq parser? Am I doing this completely wrong? (According to the multiple answered questions here on explicit left outer joins (as opposed to natural associations), I'm doing it correctly. But, it doesn't parse correctly. I've avoided creating the associations so I can join them without explicitly defining the join. Is that potentially required here and won't work properly without it?
Note: Each table has complex keys but I only really need to join based on single key values (the DB is part of a product I can't change).
Using. DotNet Core, EntityFramework, EntityFrameworkCore.SqlServer, etc., all version 1.0.1.
Help?
The short answer is to use EF6 instead of EFCore if you absolutely have to have complex Linq queries on your entites, even after the 1.1 release. There are still too many things missing in EFCore compared to EF6.
Roadmap here.
In my case, I kept EFCore and used the Context.Entity.FromSql(query) method in order to get the results. This allowed me to utilize EFCore for most of the EF Entities, and thereby keeping a forward-looking approach to the application, while allowing for special exceptions for complicated queries not based on an actual entity. The plan is to replace those FromSql queries as EF Core matures.
Prior to deciding on .FromSql, I also tested a query on a View and on a stored procedure. In both instances, I failed. For stored procedures, named parameters is not yet implemented, and views are not currently supported unless you attempt to trick EF into thinking the view is actually a table (which brings its own issues).
In order to access EF Core .FromSql, you need to install the following package:
Microsoft.EntityFrameworkCore.Relational
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 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 have a stored procedure that returns the table type. However, the EF model that has the stored procedure, only returns the present table class, but no associated classes. So when I traverse my referenced (child/parent) tables, I get nulls everywhere for those referenced table values.
I know this is expected as the stored procedure returns only that table (select * from that table), but I now would like to create a join clause AFTER calling the store procedure to get access to the rest of the referenced tables in my EF model.
Here is my code , slightly modified to make it easy to understand what I need...
var coll = db.SearchContacts(keyword,param1,param2)
//returns Contact types,works great
...now what I would like to do....(although I know syntax is off)
var ExtColl = coll join db.Address on coll.Address
join db.Department on coll.Department <- something similar
...so that I may now have access to that department (department.desc),
or address (address.civic) info from this contact.
I am at a loss as to how to implement this style of formatting AFTER I have already received it from the stored proc in the db.
Ok, so here is an update of what I am doing...
var ps = from d in db.SearchContacts(param1,param2)
select d.Id;
var p = from e in db.Contacts
.Include("Table1Ref")
.Include("Table2Ref")
.Include("Table3Ref")
Where(BuildOrExpression<DBMOdel.Contact,long>(e=>e.Id,ps.ToList()))
select e;
Now I am getting the return I want except for one thing....
there are tables that reference further referenced tables
that I would like to add, and Include does not allow
me to add them unto Contacts directly as these tables are not
referencing Contact but another table that references Contact
one layer/level further...so contacts references address
and address references province...
and I need to get access to the province info (such as description)
but need to finish my join statement properly....
So I am thinking of using join statement, but do not know the syntax for assigning or joining of this kind...
Contact -> Address on AdressId, Address -> Province on ProvinceId
SO i am thinking my linq statement would look like this...
var p = from e in db.Contacts
.Include("Table1Ref")
.Include("Table2Ref")
.Include("Table3Ref")
Where(BuildOrExpression<DBMOdel.Contact,long>(e=>e.Id,ps.ToList()))
-> join o in db.Province on o.ProvinceId equals e.Address.ProvinceId
select e;
but am not getting the desired effect...can anyone help?
I think I might be also mixing linq to sql or object with linq to entity...
this is the error I am getting for it
"The key selector type for the call to the 'Join' method is not comparable in the underlying store provider."
There is no SQL syntax to join tables to stored procedures, so EF won't be able to do this either.
Alternatives:
Do the joins in memory (linq to objects).
Do the joins in a stored procedure and map it to a complex type.
The latter option is likely to perform (much) better, but is less flexible. It's up to you to weigh the pros and cons.
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.