Informix: UPDATE with SELECT - syntax? - informix

I wanna update my table for all persons whoes activity lasted toooo long. The update should correct one time and for the subsequent rows I need to deal with new result. So thought about something like
UPDATE summary_table st
SET st.screen_on=newScreenOnValue
st.active_screen_on=st.active_screen_on-(st.screen_on-newScreenOnValue) --old-value minus thedifference
FROM (
SUB-SELECT with rowid, newScreenOnValue ... JOIN ... WHERE....
) nv
WHERE (st.rowid=nv.rowid)
I know that I can update the first and the second value directly, by rerunning the same query. But my problem is the costs of the subselect seems quite high and therefore wanna avoid a double-update resp. double-run of the same query.
The above SELECT is just a informal way of writting what I think I would like to get. I know that the st doesn't work, but I left it here for better understanding. When I try the above statement I always get back a SyntaxError at the position the FROM ends.

This can be achieved as follows:
UPDATE summary_table st
SET (st.screen_on, st.active_screen_on) =
((SELECT newScreenOnValue, st.active_screen_on-(st.screen_on-newScreenOnValue)
FROM ...
JOIN...
WHERE..))
[WHERE if any additional condition required];
The above query works perfectly fine on informix tried and tested until you make any errors in the FROM, JOIN, WHERE clauses.
Cheers !

Syntax error because a comma is missing between the first and second columns you're updating.
Never use ROWID's, they're volatile and also not used by default with IDS, unless you specify so.
Why are you using a subquery?

Related

InfluxDB query with tag where clause not returning results

this looks like something really straightforward that I am messing up, but can't find what is wrong. I have a simple query where I am trying to retrieve measurements with a specific tag value. I am following the syntax of double quoting the tag_key and single quoting the tag value. E.g., select "score","i_unique"::tag from "events.flow" where "i_unique" = 'my-value'
If you see below:
> select "score","i_unique"::tag from "events.flow" limit 4
name: events.flow
time score i_unique
---- ----- --------
1563464912039000000 462 "42422440-41124048-155896-42266768-229624-233640"
1563464912042000000 462 "42422440-41124048-155896-42266768-42352808-42270864"
......
> select "score","i_unique"::tag from "events.flow" where "i_unique" = '42422440-41124048-155896-42266768-229624-233640'
>
The tag value I want to query for, is in the time series as can be seen by the first select query. But, when I want to pick up just that value by adding a where clause, I am not getting any results.
Any help would be wonderful, Thank You.
Turns out there was an error on my insert. I was inserting an additional double quote for my tag. It is obvious now, when I look at the result of the search query, but missed it completely. So this was a problem with my insert and not select. Once I removed the additional "s, everything is making sense again.

Ascending sort order Index versus descending sort order index when performing OrderBy

I am working on an asp.net mvc web application, and I am using Sql server 2008 R2 + Entity framework.
Now on the sql server I have added a unique index on any column that might be ordered by . for example I have created a unique index on the Sql server on the Tag colum and I have defined that the sort order for the index to be Ascending. Now I have some queries inside my application that order the tag ascending while other queries order the Tag descending, as follow:-
LatestTechnology = tms.Technologies.Where(a=> !a.IsDeleted && a.IsCompleted).OrderByDescending(a => a.Tag).Take(pagesize).ToList(),;
TechnologyList = tms.Technologies.Where(a=> !a.IsDeleted && a.IsCompleted).OrderBy (a => a.Tag).Take(pagesize).ToList();
So my question is whether the two OrderByDescending(a => a.Tag). & OrderBy(a => a.Tag), can benefit from the asending unique index on the sql server on the Tag colum ? or I should define two unique indexes on the sql server one with ascending sort order while the other index with decedning sort order ?
THanks
EDIT
the following query :-
LatestTechnology = tms.Technologies.Where(a=> !a.IsDeleted && a.IsCompleted).OrderByDescending(a => a.Tag).Take(pagesize).ToList();
will generate the following sql statement as mentioned by the sql server profiler :-
SELECT TOP (15)
[Extent1].[TechnologyID] AS [TechnologyID],
[Extent1].[Tag] AS [Tag],
[Extent1].[IsDeleted] AS [IsDeleted],
[Extent1].[timestamp] AS [timestamp],
[Extent1].[TypeID] AS [TypeID],
[Extent1].[StartDate] AS [StartDate],
[Extent1].[IT360ID] AS [IT360ID],
[Extent1].[IsCompleted] AS [IsCompleted]
FROM [dbo].[Technology] AS [Extent1]
WHERE ([Extent1].[IsDeleted] <> cast(1 as bit)) AND ([Extent1].[IsCompleted] = 1)
ORDER BY [Extent1].[Tag] DESC
To answer your question:
So my question is whether the two OrderByDescending(a => a.Tag). &
OrderBy(a => a.Tag), can benefit from the asending unique index on the
sql server on the Tag colum ?
Yes, SQL Server can read an index in both directions: as in index definition or in the exact opposite direction.
However, from your intro I suspect that you still have a wrong impression how indexing works for order by. If you have both, a where clause and an order by clause, you must make sure to have a single index that covers both clauses! It does not help to have on index for the where clause (like on isDeleted and isCompleted — whatever that is in your example) and another index on tag. You need to have a single index that first has the columns of the where clause followed by the columns of the order by clause (multi-column index).
It can be tricky to make it work correctly, but it's worth the effort especially if your are only fetching the first few rows (like in your example).
If it doesn't work out right away, please have a look at this:
http://use-the-index-luke.com/sql/sorting-grouping/indexed-order-by
It is generally best to show the actual SQL query—not the .NET source code—when asking for performance advice. Then I could tell you which index to create exactly. At the moment I'm unsure about isDeleted and isCompleted — are these table columns or expressions that evaluate upon other columns?
EDIT (after you added the SQL query)
There are two ways to make your query work as indexed top-n query:
http://sqlfiddle.com/#!6/260fb/4
The first option is a regular index on the columns from the where clause followed by those from the order by clause. However, as you query uses this filter IsDeleted <> cast(1 as bit) it cannot use the index in a order-preserving way. If, however, you re-phrase the query so that it reads like this IsDeleted = cast(0 as bit) then it works. Please look at the fiddle, I've prepared everything there. Yes, SQL Server could be smart enough to know that, but it seems like it isn't.
I don't know how to tweak EF to produce the query in the above described way, sorry.
However, there is a second option using a so called filtered index — that is an index that only contains a sub-set of the table rows. It's also in the SQL Fiddle. Here it is important that you add the where clause to the index definition in the very same way as it appears in your query.
In both ways it still works if you change DESC to ASC.
The important part is that the execution plan doesn't show a sort operation. You can also verify this in SQL Fiddle (click on 'View execution plan').

Returning a semi-unique set of most recent records

In my application a User has Highlights.
Each Highlight has a HighlightType. So if I run user.highlights I might see an output like this:
Notice that there are many highlights of type_id 47. This marks milestones of the number of times the user has gone running.
What I would like to do is return this full list of records, but only include one highlight for each highlight_type, and I want that one record to be the most recent record (in this case the "50th run" highlight). So in the example above I would get the same results but with IDs 195-199 removed.
Is there an efficient way to accomplish this?
I don't think there is an easy or clean way to achieve that, nor a "Rails way". Look at e.g. this link
According to one suggestion in that link you would do this SQL request:
SELECT h1.*
FROM highlights h1
LEFT JOIN highlights h2
ON (h1.user_id = h2.user_id
AND h1.highlight_type_id = h2.highlight_type_id
AND h1.created_at < h2.created_at)
WHERE h2.id IS NULL AND h1.user_id = <the user id you are interested in>
group by h1.highlight_type_id
I think it will be some performance problem if you have big tables maybe, an it not so very clean I think.
Otherwise, if there isn't so much highlights for a user I would have done something like this:
rows = {}
user.highlights.order('highlight_type_id, created_at DESC').each do |hi|
rows[hi.highlight_type_id] ||= hi
end
# then use rows which will have one object for each highlight_type_id
The DESC on created_at is important
EDIT:
I also saw some suggestions based on this
user.highlights.group('highlight_type_id').order('created_at DESC')
And that was also how I first thought it should be solved, but I tested it and it doesn't seems to get a correct result - at least on my test data.

Return every nth row from database using ActiveRecord in rails

Ruby 1.9.2 / rails 3.1 / deploy onto heroku --> posgresql
Hi, Once a number of rows relating to an object goes over a certain amount, I wish to pull back every nth row instead. It's simply because the rows are used (in part) to display data for graphing, so once the number of rows returned goes above say 20, it's good to return every second one, and so forth.
This question seemed to point in the right direction:
ActiveRecord Find - Skipping Records or Getting Every Nth Record
Doing a mod on row number makes sense, but using basically:
#widgetstats = self.widgetstats.find(:all,:conditions => 'MOD(ROW_NUMBER(),3) = 0 ')
doesn't work, it returns an error:
PGError: ERROR: window function call requires an OVER clause
And any attempt to solve that with e.g. basing my OVER clause syntax on things I see in the answer on this question:
Row numbering in PostgreSQL
ends in syntax errors and I can't get a result.
Am I missing a more obvious way of efficiently returning every nth task or if I'm on the right track any pointers on the way to go? Obviously returning all the data and fixing it in rails afterwards is possible, but terribly inefficient.
Thank you!
I think you are looking for a query like this one:
SELECT * FROM (SELECT widgetstats.*, row_number() OVER () AS rownum FROM widgetstats ORDER BY id) stats WHERE mod(rownum,3) = 0
This is difficult to build using ActiveRecord, so you might be forced to do something like:
#widgetstats = self.widgetstats.find_by_sql(
%{
SELECT * FROM
(
SELECT widgetstats.*, row_number() OVER () AS rownum FROM widgetstats ORDER BY id
) AS stats
WHERE mod(rownum,3) = 0
}
)
You'll obviously want to change the ordering used and add any WHERE clauses or other modifications to suit your needs.
Were I to solve this, I would either just write the SQL myself, like the SQL that you linked to. You can do this with
my_model.connection.execute('...')
or just get the id numbers and find by id
ids = (1..30).step(2)
my_model.where(id => ids)

How do I use TADOQuery.Parameters with integer parameter types that have to be put in two or more places in a query?

I have a complex query that contains more than one place where the same primary key value must be substituted. It looks like this:
select Foo.Id,
Foo.BearBaitId,
Foo.LinkType,
Foo.BugId,
Foo.GooNum,
Foo.WorkOrderId,
(case when Goo.ZenID is null or Goo.ZenID=0 then
IsNull(dbo.EmptyToNull(Bar.FanName),dbo.EmptyToNull(Bar.BazName))+' '+Bar.Strength else
'#'+BarZen.Description end) as Description,
Foo.Init,
Foo.DateCreated,
Foo.DateChanged,
Bug.LastName,
Bug.FirstName,
Goo.BarID,
(case when Goo.ZenID is null or Goo.ZenID=0 then
IsNull(dbo.EmptyToNull(Bar.BazName),dbo.EmptyToNull(Bar.FanName))+' '+Bar.Strength else
'#'+BarZen.Description end) as BazName,
GooTracking.Status as GooTrackingStatus
from
Foo
inner join Bug on (Foo.BugId=Bug.Id)
inner join Goo on (Foo.GooNum=Goo.GooNum)
left join Bar on (Bar.Id=Goo.BarID)
left join BarZen on (Goo.ZenID=BarZen.ID)
inner join GooTracking on(Goo.GooNum=GooTracking.GooNum )
where (BearBaitId = :aBaitid)
UNION
select Foo.Id,
Foo.BearBaitId,
Foo.LinkType,
Foo.BugId,
Foo.GooNum,
Foo.WorkOrderId,
Foo.Description,
Foo.Init,
Foo.DateCreated,
Foo.DateChanged,
Bug.LastName,
Bug.FirstName,
0,
NULL,
0
from Foo
inner join Bug on (Foo.BugId=Bug.Id)
where (LinkType=0) and (BearBaitId= :aBaitid )
order by BearBaitId,LinkType desc, GooNum
When I try to use an integer parameter on this non-trivial query, it seems impossible to me. I get this error:
Error
Incorrect syntax near ':'.
The query works fine if I take out the :aBaitid and substitute a literal 1.
Is there something else I can do to this query above? When I test with simple tests like this:
select * from foo where id = :anid
These simple cases work fine. The component is TADOQuery, and it works fine until you add any :parameters to the SQL string.
Update: when I use the following code at runtime, the parameter substitutions are actually done (some glitch in the ADO components is worked around) and a different error surfaces:
adoFooContentQuery.Parameters.FindParam('aBaitId').Value := 1;
adoFooContentQuery.Active := true;
Now the error changes to:
Incorrect syntax near the keyword 'inner''.
Note again, that this error goes away if I simply stop using the parameter substitution feature.
Update2: The accepted answer suggests I have to find two different copies of the parameter with the same name, which bothered me so I reworked the query like this:
DECLARE #aVar int;
SET #aVar = :aBaitid;
SELECT ....(long query here)
Then I used #aVar throughout the script where needed, to avoid the repeated use of :aBaitId. (If the number of times the parameter value is used changes, I don't want to have to find all parameters matching a name, and replace them).
I suppose a helper-function like this would be fine too: SetAllParamsNamed(aQuery:TAdoQuery; aName:String;aValue:Variant)
FindParam only finds one parameter, while you have two with the same name. Delphi dataset adds each parameter as a separate one to its collection of parameters.
It should work if you loop through all parameters, check if the name matches, and set the value of each one that matches, although I normally choose to give each same parameter a follow-up number to distingish between them.

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