Suppose I have the following SPARQL query:
SELECT DISTINCT ?result ?label
WHERE {
?result a database:Column .
?result rdfs:label ?label .
}
ORDER BY (LCASE(?label))
What can I add to the query to limit the number of results to the first 10 given? Or preferably, the 10 results after the first n×10 results? I'm trying to implement pagination for visualizing the results.
I'm trying to implement a system of paging for a table that visualizes the returned data.
You want to use limit, order by, and offset. They're described pretty well in the standard:
15.4 OFFSET
OFFSET causes the solutions generated to start after the specified
number of solutions. An OFFSET of zero has no effect.
Using LIMIT and OFFSET to select different subsets of the query
solutions will not be useful unless the order is made predictable by
using ORDER BY.
15.5 LIMIT
The LIMIT clause puts an upper bound on the number of solutions
returned. If the number of actual solutions, after OFFSET is applied,
is greater than the limit, then at most the limit number of solutions
will be returned.
In your case, your query would be something like this to show the fourth page of results when you are showing ten results per page:
SELECT DISTINCT ?result ?label
WHERE {
?result a database:Column .
?result rdfs:label ?label .
}
ORDER BY (LCASE(?label))
LIMIT 10
OFFSET 30
Related
I use knexjs and postgresql. Is it possible in knexjs to get the total of records from the same query in which the limit is used?
For example:
knex.select().from('project').limit(50)
Is it possible to somehow get the total number of records in the same query if there are more than 50?
The question arose due to the fact that my query is much more complex, which uses a lot of subqueries and conditions, and I would not like to make this query twice to get the data in one query and the total number of records (I use the .count() method) from another.
I do not know your obscurification manager (knexjs?) but I would think you should be able to add the window version of the count() function to your select list. In plain SQL something like: Where ... represents your current select list. (see demo)
select ..., count(*) over() total_rows
from project
limit 5;
This works because the window count function counts all rows selected, after all rows selected, but before the LIMIT clause is applied. Note: This adds a column to the result set with the same value in every row.
I'm trying to get max drawdown from a partitioned table across multiple dates. The query works fine when run with a date constrained to a specific day. E.g.
select {max neg x-maxs x} pnl from trades where date=last date
It's getting map-reduced over multiple dates so the above query no longer works. I can make the query run over multiple dates by adding another aggregation:
select max {max neg x-maxs x} pnl from trades
but it's not getting the max drawdown from continuous sequence of trades but a maximum of daily drawdowns.
I wonder if there's a way to make it work with a single select without chaining selects like
select {max neg x-maxs x} pnl from select pnl from trades
I've got a rather big query to pull a lot of various metrics on the trades where max drawdown is just one of them. Using chained select means that I need to break the big query into two queries, map-reduced and non-map-reduced, and then join them back which would make the query look ugly.
Thanks!
Select query runs on each date in partition db and apply function to each date values and finally aggregates them depending upon the call (user defined function behaves differently than plain 'q' functions).
So I don't think you can combine that into one query. But there are ways you can look for to make your query more generalized and reusable for different scenarios.
For ex. convert your query to functional form and use variables in that query for column name and user function. Put this in one function which will accept column name and user function. Now you can call this function with different set of (column ;function). Something like :
runF:{[col;usrfunc] funtional_query_uses_col_userfunc }
All this depends on your use cases. Also check for memory usage as you'll be taking lot of data into memory.
I just started using MySQL Workbench (6.1). The default limit for queries is 1,000 and that's fine I want to keep that.
But the results from the action output message will therefore always say "1000 rows returned".
Is there a setting to see the number of records that would be returned in the query had their been no limit? For sanity checking query results?
I know this is late by a few years, but I think you're asking for a way to see total row count in the bottom of the results pane, like in SQL Server. In SQL Server, you would also go in the messages pane and it would say how many rows were returned. I was actually looking for exactly what you were asking for as well, and seems like there is no way to find that. If you have an ID in your table that is just numeric and is in numeric order, you could order by ID desc and look at the biggest number there. That is what I've decided to do.
The result is not always "1000 rows returned". If there are less records than that you will get the actual count. If you want to know the total number of rows in a table do a select count(*) from table. Alternatively, you can switch off the automatic limit and have all records returned by MySQL Workbench, but that can be time + memory consuming for large tables.
I think removing the row limit will help. By default, MySQL workbench will limit the result set to 1000 rows but you can always disable the limit. Check out https://superuser.com/questions/240291/how-to-remove-1000-row-limit-in-mysql-workbench-queries on how to do that.
You can run a second query to check that
select count(*) from (your original query) as t;
this will return the total rows in actual result.
You can use the SQL count function. It returns the count of the total number of rows a query returns.
A sample query:
select count(*) from tableName where field1 = value1
In workbench, in the dropdown menu at the top, set it to dont limit Then run the query to extract data from table Then under the output pane below, the total count of the query results will be displayed in the message column
I'm trying to fetch the n-th row of a query result. Further posts suggested the use of OFFSET or LIMIT but those forbid the use of variables (ERROR: argument of OFFSET must not contain variables). Further I read about the usage of cursors but I'm not quite sure how to use them even after reading their PostgreSQL manpage. Any other suggestions or examples for how to use cursors?
My main goal is to calculate the p-quantile of a row and since PostgreSQL doesn't provide this function by default I have to write it on my own.
Cheers
The following returns the 5th row of a result set:
select *
from (
select <column_list>,
row_number() over (order by some_sort_column) as rn
) t
where rn = 5;
You have to include an order by because otherwise the concept of "5th row" doesn't make sense.
You mention "use of variable" so I'm not sure what you are actually trying to achive. But you should be able to supply the value 5 as a variable for this query (or even a sub-select).
You might also want to dig further into windowing functions. Because with that you could e.g. do a sum() over the 3 rows before the current row (or similar constructs) - which could also be useful for you.
if you would like to get 10th record, below query also work fine.
select * from table_name order by sort_column limit 1 offset 9
OFFSET simply skip that many rows before beginning to return rows as mentioned in LIMIT clause.
We use the following pagination technique here:
get count(*) of given filter
get first 25 records of given filter
-> render some pagination links on the page
This works pretty well as long as count(*) is reasonable fast. In our case the data size has grown to a point where a non-indexd query (although most stuff is covered by indices) takes more than a minute. So at this point the user waits for a mostly unimportant number (total records matching filter, number of pages). The first N records are often ready pretty fast.
Therefore I have two questions:
can I limit the count(*) to a certain number
or would it be possible to limit it by time? (no count() known after 20ms)
Or just in general: are there some easy ways to avoid that problem? We would like to keep the system as untouched as possible.
Database: Oracle 10g
Update
There are several scenarios
a) there's an index -> neither count(*) nor the actual select should be a problem
b) there's no index
count(*) is HUGE, and it takes ages to determine it -> rownum would help
count(*) is zero or very low, here a time limit would help. Or I could just dont do a count(*) if the result set is already below the page limit.
You could use 'where rownum < x' to limit the number of rows to count. And if you need to show to your user that you has more register, you could use x+1 in count just to see if there is more than x registers.