I am using Codename One to get data from a database and display it in a table.
This is functioning quite well, and now I am trying to add a feature that limits the number of items that are shown in the table. To get there, I want to retrieve a selected number of columns from the database .
Unfortunately, common java sql operations do not seem to work. Below is what I have so far. All approaches don't throw any errors, but also also don't display the desired range or ranges. Another way to obtain a selection would probably be an ArrayList, but I would very much like to stick with a direct approach, since this appears to be relatively easy and elegant solution.
db = display.getInstance().openOrCreate("MyDB.db");
db.execute("CREATE TABLE IF NOT EXISTS Termine (Date NOT NULL,Event NOT NULL, Date_String NOT NULL)");
// prints the entire number of items from the database
cur = db.executeQuery("select * from (select * from Termine order by Date) where Date >= 5;");
// doesn't print anything
cur = db.executeQuery("SELECT * from ( select m.*, Date r from Termine m ) where r > 4 and r < 10;");
// doesn't print anything
cur = db.executeQuery("SELECT * from ( select m.*, Date r from Termine m ) where r BETWEEN 5 AND 10;");
while (cur.next()) {
Row currentRow = cur.getRow();
String event = currentRow.getString(1);
System.out.println(event);
}
Is my approach feasible? Any help would be greatly appreciated.
You can use the LIMIT SQL keyword as explained here.
SELECT column_list FROM table LIMIT row_count OFFSET offset;
A good practice is to search for "sqlite" and the SQL feature you are looking for as mobile SQL databases on all platforms are based on sqlite which has some quirks.
Related
I'm trying to join two continuous queries, but keep running into the following error:
Rowtime attributes must not be in the input rows of a regular join. As a workaround you can cast the time attributes of input tables to TIMESTAMP before.\nPlease check the documentation for the set of currently supported SQL features.
Here's the table definition:
CREATE TABLE `Combined` (
`machineID` STRING,
`cycleID` BIGINT,
`start` TIMESTAMP(3),
`end` TIMESTAMP(3),
WATERMARK FOR `end` AS `end` - INTERVAL '5' SECOND,
`sensor1` FLOAT,
`sensor2` FLOAT
)
and the insert query
INSERT INTO `Combined`
SELECT
a.`MachineID`,
a.`cycleID`,
MAX(a.`start`) `start`,
MAX(a.`end`) `end`,
MAX(a.`sensor1`) `sensor1`,
MAX(m.`sensor2`) `sensor2`
FROM `Aggregated` a, `MachineStatus` m
WHERE
a.`MachineID` = m.`MachineID` AND
a.`cycleID` = m.`cycleID` AND
a.`start` = m.`timestamp`
GROUP BY a.`MachineID`, a.`cycleID`, SESSION(a.`start`, INTERVAL '1' SECOND)
In the source tables Aggregated and MachineStatus, the start and timestamp columns are time attributes with a watermark.
I've tried casting the input rows of the join to timestamps, but that didn't fix the issue and would mean that I cannot use SESSION, which is supposed to ensure that only one data point gets recorded per cycle.
Any help is greatly appreciated!
I investigated this a little further and noticed that the GROUP BY statement doesn't make sense in that context.
Furthermore, the SESSION can be replaced by a time window, which is the more idiomatic approach.
INSERT INTO `Combined`
SELECT
a.`MachineID`,
a.`cycleID`,
a.`start`,
a.`end`,
a.`sensor1`,
m.`sensor2`
FROM `Aggregated` a, `MachineStatus` m
WHERE
a.`MachineID` = m.`MachineID` AND
a.`cycleID` = m.`cycleID` AND
m.`timestamp` BETWEEN a.`start` AND a.`start` + INTERVAL '0' SECOND
To understand the different ways to join dynamic tables, I found the Ververica SQL training extremely helpful.
Recently I ran into an issue where we have multiple concurrent client requests causing performance issue in db. I tried the test scenario and as it turned out, when I run SELECT queries (same query) 6 to 7 times (gets worse with more), It degrades the performance and execution takes a lot of time. However I tried this one
SELECT TOP (100) COUNT(DISTINCT([Doc_Number])) AS "Expression"
FROM (
SELECT *
FROM "dbo"."Dummy_Table" "table_alias"
WHERE ((CAST("table_alias"."ID" AS NVARCHAR)) NOT IN
(
SELECT "PrimaryKey" AS ExceptionKey
FROM dbo.exceptions inner_exceptionStatus
LEFT JOIN dbo.Workflow inner_workflowStates ON
(inner_exceptionStatus."Status"= inner_workflowStates."UUID" AND
inner_exceptionStatus."UUID"= 'CA1662D6-73A2-4692-A765-E7E3EDB66062')
WHERE ("inner_workflowStates"."RemoveFromRecordSet" = 1 AND
"inner_workflowStates"."IsDeleted" = 0) AND
("inner_exceptionStatus"."IsArchived" IS NULL OR
"inner_exceptionStatus"."IsArchived" = 0)))) wrapperQuery
The query when runs alone takes around 1sec execution time. But If we runs it in parallel, for each query it takes up a wried amount of time of leads to timeout.
The only thing bothers me here is that SELECT query should be non-blocking and even with shared lock, then need to get along easily.
I am not sure if there is anything wrong in the query that adds up the situation.
Any help is deeply appreciated !!
Try this way
SELECT Count(DISTINCT( [Doc_Number] )) AS Expression
FROM dbo.Dummy_Table table_alias
WHERE NOT EXISTS (SELECT 1
FROM dbo.exceptions inner_exceptionStatus
INNER JOIN dbo.Workflow inner_workflowStates
ON ( inner_exceptionStatus.Status = inner_workflowStates.UUID
AND inner_exceptionStatus.UUID = 'CA1662D6-73A2-4692-A765-E7E3EDB66062' )
WHERE inner_workflowStates.RemoveFromRecordSet = 1
AND inner_workflowStates.IsDeleted = 0
AND ( inner_exceptionStatus.IsArchived IS NULL
OR inner_exceptionStatus.IsArchived = 0 )
AND table_alias.ID = PrimaryKey)
Made couple of changes.
Changed NOT IN to NOT EXISTS
Removed the convert in "table_alias"."ID" because it will avoid using any index present in "table_alias"."ID" column. If the conversion is really required then add it.
Removed Top (100) since there is no Group By it will return a single record as result.
Still if the query is running slow then you need to post the execution plan and make sure the statistics are up-to-date
You can simplyfy your query like this :
SELECT COUNT(DISTINCT(Doc_Number)) AS Expression
FROM dbo.Dummy_Table dmy
WHERE not exists
(
SELECT *
FROM dbo.exceptions ies
INNER JOIN dbo.Workflow iws ON ies.Status= iws.UUID AND ies.UUID= 'CA1662D6-73A2-4692-A765-E7E3EDB66062'
WHERE iws.RemoveFromRecordSet = 1 AND iws.IsDeleted = 0 AND (ies.IsArchived IS NULL OR ies.IsArchived = 0)
and dmy.ID=PrimaryKey
)
Like prdp say :
Changed NOT IN to NOT EXISTS
Removed the convert in "table_alias"."ID" because it will avoid using any index present in "table_alias"."ID" column. If the conversion is really required then add it.
Removed Top (100) since there is no Group By it will return a single record as result.
I add :
Remove you temporary table wrapperQuery
You can use INNER JOIN because into where you test RemoveFromRecordSet = 1 then you remove null values.
Remove not utils quotes ,brackets and parenthèses into where clause
I've got a question in terms of processing and making a query more efficient whilst maintaining its accuracy. Before I display the query I'd like to point out some basics of it.
I've got a case that manipulates the where-clause to get all childs of the parent. Basically I've got two types of data that I need to display; a red and a green type. The red type has a column (TRK_TrackerGroup_LKID2) set to NULL by default, whereas the green data has a value in said column (ranging from 5-7).
My problem is that I need to extract both types of data to accurately get a count of outstanding issues in a view, but doing so (by adding the case) the execution time goes from < 1 second to well over 15 seconds.
This is the query (with the mentioned case):
SELECT TS.id AS TrackerStartDateID,
TSM.mappingtypeid,
TSM.maptoid,
TFLK.trk_trackergroup_lkid,
Count(TF.id) AS Cnt
FROM [dbo].[trk_startdate] TS
INNER JOIN [dbo].[trk_startdatemap] TSM
ON TS.id = TSM.trk_startdateid
AND TSM.deletedflag = 0
INNER JOIN [dbo].[trk_trackerfeatures] TF
ON TF.trk_startdateid = TS.id
AND TF.deletedflag = 0
INNER JOIN [dbo].[trk_trackerfeatures_lk] TFLK
ON TFLK.id = TF.trk_feature_lkid
WHERE TS.deletedflag = 0
AND TF.applicabletoproject = 1
AND TF.readyforwork = CASE -- HERE IS THE PROBLEM
WHEN TF.trk_trackerstatus_lkid2 IS NULL THEN 0
ELSE 1
END
AND TF.datestamp = (SELECT Max(TF2.datestamp)
FROM [dbo].[trk_trackerfeatures] TF2
INNER JOIN [dbo].[trk_trackerfeatures_lk] TFLK2
ON TFLK2.id = TF2.trk_feature_lkid
WHERE TF.trk_startdateid = TF2.trk_startdateid
AND TFLK2.trk_trackergroup_lkid = TFLK.trk_trackergroup_lkid)
GROUP BY TS.id,
TSM.mappingtypeid,
TSM.maptoid,
TFLK.trk_trackergroup_lkid,
TF.datestamp
It functions as a 'parent' in the sense that it grabs the latest inserted data-set (using DateStamp) from every single child-group. This is necessary to produce a parent-report in SSRS report at a later time, but at the moment my problem (as mentioned above) is the execution time.
I'd like to hear if there are any suggestions on how to decrease the execution time whilst maintaining the accuracy of the query.
Expected output:
Without the case I get this:
Your problem is this condition cant use INDEX
AND TF.readyforwork = CASE -- HERE IS THE PROBLEM
WHEN TF.trk_trackerstatus_lkid2 IS NULL THEN 0
ELSE 1
END
Try to change it to
AND ( TF.readyforwork = 0 and TF.trk_trackerstatus_lkid2 IS NULL
OR TF.readyforwork = 1 and TF.trk_trackerstatus_lkid2 IS NOT NULL
)
But again you should check with EXPLAIN ANALIZE to test if your query is using index or not.
The most problematic bit of your query seems to be the correlated subquery, because you must call it for every possible row.
You should optimize this first. To do so you can add indexes that the engine could use to quickly calculate that value on each row.
Based on your query I would add these two indexes multiples :
On Table trackerfeatures, index fields : trk_startdateid, datestamp
On Table trk_trackerfeatures_lk, index fields : id, trk_trackergroup_lkid
I have a search screen which allows the user to search information and it populates a grid. Some of the search results are returning a huge amount of data. I am trying to create a paging grid so that I only bring from the store procedure 10 or 20 results at a time. (I already have a paging grid in the UI)
I am trying to do something like this:
select * from wl_eval limit 1, 20
The query above will return only the first 20 records. How would I be able to accomplish that in Sybase ASE? Unfortunately, for my client project we are using Sybase. I know that in other database engines we could have used the query I mention above. I also know we can use SET ROW COUNT 20 but this wont work if I want a particular range, say from 30 to 50.
Any thoughts?
Add TOP numberofrecords you want to return
DECLARE #intStartRow int;
DECLARE #intEndRow int;
SET #intStartRow = (#intPage -1) * #intPageSize + 1;
SET #intEndRow = #intPage * #intPageSize;
WITH wl_eval AS
(SELECT field,
ROW_NUMBER() OVER(ORDER BY intID DESC) as intRow,
COUNT(intID) OVER() AS intTotalHits
FROM tblBlog)
SELECT field, intTotalHits FROM wl
WHERE intRow BETWEEN #intStartRow AND #intEndRow
SELECT TOP 20 <column list>
FROM YourTable
ORDER BY <list>
local function getListRecord(id)
local select_stmt = db:prepare("SELECT * FROM list where id = ?")
select_stmt:bind_values(id)
select_stmt:step()
local r = select_stmt:get_uvalues()
select_stmt:finalize()
return r
end
for the above code i am getting duplicate record created. how can i return single record
Please help me, thanks in advance
Thank you,
Madan mohan.
Use of DISTINCT is one way. Another may be to use LIMIT 1. The syntax and options may vary depending on which database you run the query against.
Examples:
SELECT DISTINCT * FROM list where id = ?
SELECT * FROM list where id = ? LIMIT 1