I am looking for the way to execute MySQL statement checking if given parameter exists. As I remember I can do the following in Oracle to achieve that:
select s.* from Site s
where s.study = :study
and (:enabled is null or s.enabled = :enabled)
is anything like that possible in MySQL too? The same code executes without error but never return any records.
My goal here is to avoid multiple lfs and elses in my java code. It should work the way that the query looks like that when enabled parameter is null:
select s.* from Site s
where s.study = :study
and like that if the parameter is not null:
select s.* from Site s
where s.study = :study
and s.enabled = :enabled
and I want to do that with a single query
I believe this is what you are asking:
SELECT s.* from Site s
WHERE s.study = "some_study"
AND (s.enabled IS NULL OR s.enabled = '' OR s.enabled = "enabled");
Unfortunately it is highly dependent on database driver. My initial query works when run in database tools but doesn't have to when it comes to run it by JPA. So I'm to close this question as it doesn't require further answers. I'm sorry lads for wasting your time.
I have two tables.
(1) compressors
(2) crankcase_heaters
I'm trying to create a SQL query to do:
Select the compressor.VOLTAGE and compressor.WATT of each compressor.PART_NUMBER
Find the crankcase_heater.PARTNO that has the same voltage and watts.
Add that value into a new field on the compressor table called "CRANKHTR"
Essentially this query will reproduce my compressors table but will have another 'column' called "CRANKHTR".
I'm completely lost on where to even start with this. I tried using the phpmyadmin SQL Query builder but i have no idea where to begin.
Without seeing the exact data structure, it sounds like you need a simple INNER JOIN:
SELECT
`cp`.`VOLTAGE`,
`cp`.`WATT`,
`ch`.`PARTNO` as CRANKHTR
FROM
`compressor` cp
INNER JOIN `crankcase_heaters` ch ON ch.VOLTAGE = cp.VOLTAGE AND ch.WATT = cp.WATT
I have a Mysql query which is this:
actors_to_delete = find_by_sql("SELECT * FROM `dvd_role` a
LEFT JOIN `dvd_actor2role` b ON a.id = b.`roleId`
LEFT JOIN `dvd_actor` c ON b.`actorId` = c.`id`
WHERE role LIKE '%uncredited%'
GROUP BY c.id")
I've written it the Railsy way like this:
actors_to_delete = Role.joins("LEFT JOIN `dvd_actor2role` ON dvd_role.id = dvd_actor2role.`roleId`").joins("LEFT JOIN `dvd_actor` ON dvd_actor2role.`actorId` = dvd_actor.`id`").where("dvd_role.role LIKE '%uncredited%'").group("dvd_actor.id")
What I'm wondering (apart from the fact that what is really the difference between these 2 queries, they both do the same thing, so why favour the Railsy way over the more straightforward sql way?) is how do I write it with the relations that are already established in Rails.
If I try do something like this:
Actor.actor2role.role.where("role LIKE ?", '%uncredited%')
I'll get undefined method actor2role because, even though the relationships have been established in Rails, they work for an instance of Actor, not the model itself.
So, in conclusion, just wondering what the best way to do it is. Coming from PHP and Mysql I tend to write a lot of these things out the sql way and am trying to change my evil ways :)
Edit
I also have another problem and that's the fact that in the sql query I get all the info from all 3 tables, the Rails way gives me only the dvd_role table for some reason.
How can I get the data from the other 2 table as well?
I was able to do the latter by adding .select("*") in the beginning. Is this the appropriate way:
actors_to_delete = Role.select("*").joins("LEFT JOIN `dvd_actor2role` ON dvd_role.id = dvd_actor2role.`roleId`").joins("LEFT JOIN `dvd_actor` ON dvd_actor2role.`actorId` = dvd_actor.`id`").where("dvd_role.role LIKE '%uncredited%'").group("dvd_actor.id")
I want to get items from one container that don't exist in another. One container is IEnumerable, and another is an entity in DB. For example
IEnumberable<int> ids = new List<int>();
ids.Add(1);
ids.Add(2);
ids.Add(3);
using (MyObjectContext ctx = new MyObjectContext())
{
var filtered_ids = ids.Except(from u in ctx.Users select u.id);
}
This approach works, but I realized that underlying sql is something like SELECT id FROM [Users]. That is not what I want. Changing it to
var filtered_ids = ids.Except(from u in ctx.Users
where ids.Contains(u.id)
select u.id);
improves underlying query and adds WHERE [id] IN (...) which seems a way better.
I have 2 questions:
Is it possible to improve performance any further for this query?
As far as I remember there is a limit on how many parameters can be in IN . Will my current query work if I exceed the limit (which is not very likely to happen, but it's better to be prepare) ?
That query should be fine, provided proper indexes/primary keys are in place.
The upper limit on sql parameters accepted by sql server is around 2100. If you exceed the limit, you will be met with a sql exception instead of results.
I'm using LINQ to Entities to display paged results. But I'm having issues with the combination of Skip(), Take() and OrderBy() calls.
Everything works fine, except that OrderBy() is assigned too late. It's executed after result set has been cut down by Skip() and Take().
So each page of results has items in order. But ordering is done on a page handful of data instead of ordering of the whole set and then limiting those records with Skip() and Take().
How do I set precedence with these statements?
My example (simplified)
var query = ctx.EntitySet.Where(/* filter */).OrderByDescending(e => e.ChangedDate);
int total = query.Count();
var result = query.Skip(n).Take(x).ToList();
One possible (but a bad) solution
One possible solution would be to apply clustered index to order by column, but this column changes frequently, which would slow database performance on inserts and updates. And I really don't want to do that.
EDIT
I ran ToTraceString() on my query where we can actually see when order by is applied to the result set. Unfortunately at the end. :(
SELECT
-- columns
FROM (SELECT
-- columns
FROM (SELECT -- columns
FROM ( SELECT
-- columns
FROM table1 AS Extent1
WHERE EXISTS (SELECT
-- single constant column
FROM table2 AS Extent2
WHERE (Extent1.ID = Extent2.ID) AND (Extent2.userId = :p__linq__4)
)
) AS Project2
limit 0,10 ) AS Limit1
LEFT OUTER JOIN (SELECT
-- columns
FROM table2 AS Extent3 ) AS Project3 ON Limit1.ID = Project3.ID
UNION ALL
SELECT
-- columns
FROM (SELECT -- columns
FROM ( SELECT
-- columns
FROM table1 AS Extent4
WHERE EXISTS (SELECT
-- single constant column
FROM table2 AS Extent5
WHERE (Extent4.ID = Extent5.ID) AND (Extent5.userId = :p__linq__4)
)
) AS Project6
limit 0,10 ) AS Limit2
INNER JOIN table3 AS Extent6 ON Limit2.ID = Extent6.ID) AS UnionAll1
ORDER BY UnionAll1.ChangedDate DESC, UnionAll1.ID ASC, UnionAll1.C1 ASC
My workaround solution
I've managed to workaround this problem. Don't get me wrong here. I haven't solved precedence issue as of yet, but I've mitigated it.
What I did?
This is the code I've used until I get an answer from Devart. If they won't be able to overcome this issue I'll have to use this code in the end.
// get ordered list of IDs
List<int> ids = ctx.MyEntitySet
.Include(/* Related entity set that is needed in where clause */)
.Where(/* filter */)
.OrderByDescending(e => e.ChangedDate)
.Select(e => e.Id)
.ToList();
// get total count
int total = ids.Count;
if (total > 0)
{
// get a single page of results
List<MyEntity> result = ctx.MyEntitySet
.Include(/* related entity set (as described above) */)
.Include(/* additional entity set that's neede in end results */)
.Where(string.Format("it.Id in {{{0}}}", string.Join(",", ids.ConvertAll(id => id.ToString()).Skip(pageSize * currentPageIndex).Take(pageSize).ToArray())))
.OrderByDescending(e => e.ChangedOn)
.ToList();
}
First of all I'm getting ordered IDs of my entities. Getting only IDs is well performant even with larger set of data. MySql query is quite simple and performs really well. In the second part I partition these IDs and use them to get actual entity instances.
Thinking of it, this should perform even better than the way I was doing it at the beginning (as described in my question), because getting total count is much much quicker due to simplified query. The second part is practically very very similar, except that my entities are returned rather by their IDs instead of partitioned using Skip and Take...
Hopefully someone may find this solution helpful.
I haven't worked directly with Linq to Entities, but it should have a way to hook specific stored procedures into certain locations when needed. (Linq to SQL did.) If so, you could turn this query into a stored procedure, doing exacly what is required, and doing it efficiently.
Assuming from you comment the persisting the values in a List is not acceptable:
There's no way to completely minimize the iterations, as you intended (and as I would have tried too, living in hope). Cutting the iterations down by one would be nice. Is it possible to just get the Count once and cache/session it? Then you could:
int total = ctx.EntitySet.Count; // Hopefully you can not repeat doing this.
var result = ctx.EntitySet.Where(/* filter */).OrderBy(/* expression */).Skip(n).Take(x).ToList();
Hopefully you can cache the Count somehow, or avoid needing it every time. Even if you can't, this is the best you can do.
Could you please create a sample illusrating the problem and send it to us (support * devart * com, subject "EF: Skip, Take, OrderBy")?
Hope we will be able to help you.
You can also contact us using our forums or contact form.
Are you absolutely certain the ordering is off? What does the SQL look like?
Can you reorder your code as follows and post the output?
// Redefine your queries.
var query = ctx.EntitySet.Where(/* filter */).OrderBy(e => e.ChangedDate);
var skipped = query.Skip(n).Take(x);
// let's look at the SQL, shall we?
var querySQL = query.ToTraceString();
var skippedSQL = skipped.ToTraceString();
// actual execution of the queries...
int total = query.Count();
var result = skipped.ToList();
Edit:
I'm absolutely certain. You can check my "edit" to see trace result of my query with skipped trace result that is imperative in this case. Count is not really important.
Yeah, I see it. Wow, that's a stumper. Might even be an outright bug. I note you're not using SQL Server... what DB are you using? Looks like it might be MySQl.
One way:
var query = ctx.EntitySet.Where(/* filter */).OrderBy(/* expression */).ToList();
int total = query.Count;
var result = query.Skip(n).Take(x).ToList();
Convert it to a List before skipping. It's not too efficient, mind you...