I have one table, 'a', with id and timestamp. Another table, 'b', has N multiple rows referring to id, and each row has 'type', and "some other data".
I want a LINQ query to produce a single row with id, timestamp, and "some other data" x N. Like this:
1 | 4671 | 46.5 | 56.5
where 46.5 is from one row of 'b', and 56.5 is from another row; both with the same id.
I have a working query in SQLite, but I am new to LINQ. I dont know where to start - I don't think this is a JOIN at all.
SELECT
a.id as id,
a.seconds,
COALESCE(
(SELECT b.some_data FROM
b WHERE
b.id=a.id AND b.type=1), '') AS 'data_one',
COALESCE(
(SELECT b.some_data FROM
b WHERE
b.id=a.id AND b.type=2), '') AS 'data_two'
FROM a first
WHERE first.id=1
GROUP BY first.ID
you didn't mention if you are using Linq to sql or linq to entities. However following query should get u there
(from x in a
join y in b on x.id equals y.id
select new{x.id, x.seconds, y.some_data, y.type}).GroupBy(x=>new{x.id,x.seconds}).
Select(x=>new{
id = x.key.id,
seconds = x.Key.seconds,
data_one = x.Where(z=>z.type == 1).Select(g=>g.some_data).FirstOrDefault(),
data_two = x.Where(z=>z.type == 2).Select(g=>g.some_data).FirstOrDefault()
});
Obviously, you have to prefix your table names with datacontext or Objectcontext depending upon the underlying provider.
What you want to do is similar to pivoting, see Is it possible to Pivot data using LINQ?. The difference here is that you don't really need to aggregate (like a standard pivot), so you'll need to use Max or some similar method that can simulate selecting a single varchar field.
Related
I have a query which uses the view a as follows and the query is extremely slow.
select *
from a
where a.id = 1 and a.name = 'Ann';
The view a is made up another four views b,c,d,e.
select b.id, c.name, c.age, e.town
from b,c,d,e
where c.name = b.name AND c.id = d.id AND d.name = e.name;
I have created an index on the table of c named c_test and I need to use it when executing the first query.
Is this possible?
Are you really using this deprecated 1980s join syntax? You shouldn't. Use proper explicit joins (INNER JOIN in your case).
You are joining the two tables C and D on their IDs. That should mean they are 1:1 related. If not, "ID" is a misnomer, because an ID is supposed to identify a row.
Now let's look at the access route: You have the ID from table B and the name from tables B and C. We can tell from the column name that b.id is unique and Oracle guarantees this with a unique index, if the database is set up properly.
This means the DBMS will look for the B row with ID 1, find it instantly in the index, find the row instantly in the table, see the name and see whether it matches 'Ann'.
The only thing that can be slow hence is joining C, D, and E. Joining on unique IDs is extremely fast. Joining on (non-unigue?) names is only fast, if you provide indexes on the names. I'd recommend the following indexes accordingly:
create index idx_c on c (name);
create index idx_e on e (name);
To get this faster still, use covering indexes instead:
create index idx_b on b (id, name);
create index idx_c on c (name, id, age);
create index idx_d on d (id, name);
create index idx_e on e (name, town);
I created this query using HQL with Hibernate and Oracle
select c from Cat c
left join c.kittens k
where (c.location= 1 OR c.location = 2)
and (i.activo = 1)
group
by c.id,
c.name,
c.fulldescription,
c.kittens
order by count(e) desc
The problem comes with the fact that in HQL you need to specify all fields in Cat in order to perform a Group By, but fulldescription is a CLOB, and you cannot group by by a CLOB (I get a "Not a Group By Expression" error. I've seen a few solutions around for a pure SQL sentence but none for HQL.
A serious issue GROUP BY of HQL because if you specify your object in GROUP BY and in your SELECT field list behaviours are differents. In GROUP BY has considered only id field but in SELECT field list all fields are considered.
So you can use a subquery with GROUP BY to return only id from your object, so that result becomes an input for the main query, like the follow I write for you.
Pay attention there are some alias table (i and e) not defined, so this query doesn't work, but you know as fixed.
Try this:
select c2 from Cat c2
where c2.id in (
select c.id from Cat c
left join c.kittens k
where (c.location= 1 OR c.location = 2)
and (i.activo = 1) <-- who is i alias??
group by c.id)
order by count(e) desc <-- who is e alias???
I have a table something like this:
ID|Value
01|1
02|4
03|12
01|5
02|14
03|22
01|9
02|32
02|62
01|13
03|92
I want to know how much progress have each id made (from initial or minimal value)
so in sybase I can type:
select ID, (value-min(value)) from table group by id;
ID|Value
01|0
01|4
01|8
01|12
02|0
02|10
02|28
02|58
03|0
03|10
03|80
But monetdb does not support this (I am not sure may be cz it uses SQL'99).
Group by only gives one column or may be average of other values but not the desired result.
Are there any alternative to group by in monetdb?
You can achieve this with a self join. The idea is that you build a subselect that gives you the minimum value for each id, and then join that to the original table by id.
SELECT a.id, a.value-b.min_value
FROM "table" a INNER JOIN
(SELECT id, MIN(value) AS min_value FROM "table" GROUP BY id) AS b
ON a.id = b.id;
I have a list of strings (converted from Guid) that contains the ID's of items I want to pull from my table.
Then, in my LINQ query, I am trying to figure out how to do an in clause to pull records that are in that list.
Here is the LINQ
var RegionRequests = (from r in db.course_requests
where PendingIdList.Contains(r.request_state.ToString())
select r).ToList();
It builds, but I get a run error: "System.NotSupportedException: LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression".
I would prefer to compare guid to guid, but that gets me nowhere.
Can this be converted to a lambda expression? If that is best, how?
LINQ to Entites tries to convert your expression to an SQL Statement. Your server didn't know the stored procedure ToString().
Fix:
var regionRequests =
from r in db.course_requests.ToList()
where PendingIdList.Contains(r.request_state.ToString())
select r;
With db.course_requests.ToList() you force LINQ to materialize your database data (if big table, you gonna have a bad time) and the ToString() is executed in the object context.
You stated: I have a list of strings (converted from Guid) ...
Can you NOT convert them into strings and keep it as a List< System.Guid>?? Then you can do this (assuming PendingIdGuidList is List< System.Guid>:
var regionRequets = (from r in db.course_requests
join p in PendingIdGuidList on u.request_state equals p
select r).ToList();
Edited to add:
I ran a test on this using the following code:
var db = new EntityModels.MapleCreekEntities();
List<System.Guid> PendingIdGuidList =
new List<System.Guid>() {
System.Guid.Parse("77dfd79e-2d61-40b9-ac23-36eb53dc55bc"),
System.Guid.Parse("cd409b96-de92-4fd7-8870-aa42eb5b8751")
};
var regionRequets = (from r in db.Users
join p in PendingIdGuidList on r.Test equals p
select r).ToList();
Users is a table in my database. I added a column called Test as a Uniqueidentifier data type, then modified 2 records with the following Guids.
I know it's not exactly a 1:1 of what the OP is doing, but pretty close. Here is the profiled SQL statement:
SELECT
[Extent1].[ID] AS [ID],
[Extent1].[UserLogin] AS [UserLogin],
[Extent1].[Password] AS [Password],
[Extent1].[Test] AS [Test]
FROM [dbo].[Users] AS [Extent1]
INNER JOIN (SELECT
cast('77dfd79e-2d61-40b9-ac23-36eb53dc55bc' as uniqueidentifier) AS [C1]
FROM ( SELECT 1 AS X ) AS [SingleRowTable1]
UNION ALL
SELECT
cast('cd409b96-de92-4fd7-8870-aa42eb5b8751' as uniqueidentifier) AS [C1]
FROM ( SELECT 1 AS X ) AS [SingleRowTable2]) AS [UnionAll1] ON [Extent1].[Test] = [UnionAll1].[C1]
I have 2 tables which in simplified form look like this:
Products(
id: int,
name: varchar
);
ProductSpecs(
product_id: int,
spec_name: varchar,
spec_value: int
);
Now I need to sort products (in linq to sql) by value of some specification item (eg. "price"). So I do something like this
var products = from p in db.Products
from ps in p.ProductsSpecs
where ps.spec_name == "price"
orderby ps.spec_value
select p;
The problem is that if there's no such ProductSpec with spec_name "price" the product is not included at all. I can add these products with Union or Concat but this way the sorting of the first part is not preserved.
What is the best way to deal with this?
Thanks.
First, I would recommend that you either do this in pure SQL as a function or Stored Procedure and then access this through linq, or add a price column to your product table. It seems like price would be a normal attribute to add to all of your products even if that price is NULL.
SQL:
select p.*
from products p
left outer join productspecs ps on
p.id = ps.product_id
and ps.spec_name = 'Price'
order by ps.spec_value
With that said, here's the weird bit of LINQ that should work on your table (I might have some of the column names spelled incorrectly):
var products = from p in db.Products
join ps in (from pss in db.ProductSpecs
where pss.spec_name== "Price"
select pss
) on p.id equals ps.product_id into temp
from t in temp.DefaultIfEmpty()
orderby t.spec_value
select p;
I tested this on some tables setup like above and created 5 products, three with prices in different value orders and this LINQ ordered them just like the SQL above and returned the null result rows as well.
Hope this works!
In ordinary SQL, you'd use an LEFT OUTER JOIN. This preserves rows that appear in the left-hand table (the one listed first), even when there's no matching row in the right-hand table (the second one listed, and the one that is outer joined). You end up with nulls for the values that should be, but weren't, present in the right-hand table. So, the price for those items missing a price would appear as NULL.
What that translates to in LINQ to SQL is another matter.
You might care to think about whether it is reasonable to have products that do not have a price. You're emulating something called EAV - Entity, Attribute, Value - tables, and they are generally regarded as 'not a good thing'.
Can you not just do a simple join?
var products =
from p in db.Products
join ps in db.ProductSpecs on p.id equals ps.product_id
where ps.spec_name == "price"
orderby ps.spec_value
select p;