Question
Say we have 1 as foo, and we want foo+1 as bar in SQL.
With CTE or subquery, like:-
select foo+1 as bar from (select 1 as foo) as abc;
We would get (in postgre which is what I am using):-
bar
-----
2
However, when I tried the following:-
select 1 as foo, foo+1 as bar;
The following error occurs:-
ERROR: column "foo" does not exist
LINE 1: select 1 as foo, foo+1 as bar;
^
Is there any way around this without the use of CTE or subquery?
Why do I ask?
I am using Django for a web service, to order and paginate objects in the database, I have to grab the count of the upvotes and downvotes and do some extra mathematical manipulation on those two values (ie. calculating the wilson score interval), where those two values are used multiple times.
All I can work with that I know of right now is the extra() function without breaking the ORM(?) [for example lazy queryset and prefetch_related() function].
Therefore I need a way to call those two values from somewhere instead of doing a SELECT multiple times when I calculate the score. (Or that's not the case in reality anyway?)
PS. Currently I am storing the vote count as database field and update them, but I already have a model of a vote, so it seems redundant and slow to update vote count and insert vote to database
No, you need the sub-query or CTE to do that. There is one alternative though: create a stored procedure.
CREATE FUNCTION wilson(upvote integer, downvote integer) RETURNS float8 AS $$
DECLARE
score float8;
BEGIN
-- Calculate the score
RETURN score;
END; $$ LANGUAGE plpgsql STRICT;
In your ORM you now call the function as part of your SELECT statement:
SELECT id, upvotes, downvotes, wilson(upvotes, downvotes) FROM mytable;
Also makes for cleaner code.
Related
The challenge is to update a table by scanning that same table for information. In this case, I want to find how many entries I received in an Upload dataset that have the same key (effectively duplicate instructions).
I tried the obvious code:
UPDATE Base AS TAR
INNER JOIN (select cdKey, count(*) as ct FROM Base GROUP BY cdKey) AS CHK
ON TAR.cdKey = CHK.cdKey
SET ctReferences = CHK.ct
This resulted in a non-updateable complaint. Some workarounds talked about adding DISTINCTROW, but that made no difference.
I tried creating a view (query in Ms/Access parlance); same failure.
Then I projected the set (SELECT cdKey, count(*) INTO TEMP FROM Base GROUP BY cdKey), and substituted TEMP for the INNER JOIN which worked.
Conclusion: reflexive updates are also non-updateable.
An initial thought was to embed a sub-select in the update, for example:
UPDATE Base TAR SET TAR.ctReferences = (select count(*) from Base CHK where CHK.cd = TAR.cd)
This also failed.
As this is part of a job I am calling, this SQL (like the other statements) are all strings executed by CurrentDb.Execute statements. I thought maybe I could make this a DLookup, I found that as cd is a string, I had a gaggle of double- and triple-quoted elements that was too messy to read (and maintain).
Best solution was to write a function so I could avoid having to do any sort of string manipulation. Hence, in a module there's a function:
Public Function PassHelperCtOccurs(ByRef cdX As String) As Long
PassHelperCtOccurs = DLookup("count(*)", "Base", "cd='" & cdX & "'")
End Function
And the call is:
CurrentDb().Execute ("UPDATE Base SET ctOccursCd =PassHelperCtOccurs(cd)")
I have the following query in M:
= Table.Combine({
Table.Distinct(Table.SelectColumns(Tab1,{"item"})),
Table.Distinct(Table.SelectColumns(Tab2,{"Column1"}))
})
Is it possible to get it working without prior changing column names?
I want to get something similar to SQL syntax:
select item from Tab1 union all
select Column1 from Tab2
If you need just one column from each table then you may use this code:
= Table.FromList(List.Distinct(Tab1[item])
& List.Distinct(Tab2[Column1]))
If you use M (like in your example or the append query option) the columns names must be the same otherwise it wont work.
But it works in DAX with the command
=UNION(Table1; Table2)
https://learn.microsoft.com/en-us/dax/union-function-dax
It's not possible in Power Query M. Table.Combine make an union with columns that match. If you want to keep all in the same step you can add the change names step instead of tap2 like you did with Table.SelectColumns.
This comparison of matching names is to union in a correct way.
Hope you can manage in the same step if that's what you want.
For an assignment, I'm looking to make my code faster. I'm using the sqlite3 c++ API to perform tasks in order to eventually build an r-tree and b-tree.
I am doing the assignment's tasks correctly, but unfortunately it's extremely slow. For my question, I'll first show simple mock tables, then show a simple flow of my program.
Simplified table schema's:
areaTable (id int, closed int)
middleTable (nodeid int, areaid int)
nodeTable (id int, x float, y float)
The flow of my program is as follows:
query1
SELECT id FROM areaTable WHERE closed = 1;
Using query1 I save the resulting id's into an vector array (we'll call it query1ResultsArray).
Then using sqlite3_prepare_v2 I prepare a new select query:
query2
SELECT MIN(x), MIN(y)
FROM nodeTable
WHERE id IN
(
SELECT nodeid
FROM middleTable
WHERE areaid = ?
);
The idea of query 2 is that we find will find the minimum values of the nodes that get grouped together by middleTable and areaTable. I bind individual results from query1 into query2 using a for loop like the following:
prepare query2
begin transaction (not sure if this helps)
for (auto &id : query1ResultsArray) {
bind(id)
step(stmt)
x = column 0
y = column 1
cout << "INSERT INTO ...."
reset(stmt)
}
end transaction
finalize(stmt)
This solution appears to work. It get's the proper results I need to continue with the assignment's tasks (building insert statements), but it's very very slow. I doubt the professor expects our programs to be this slow.
This was context for my question. The question itself is essentially:
Am I able to combine my two select statements? By combining the select statements I would be able to circumvent the constant binding and resetting which I hope (with no knowledge to back it up) will speed up my program.
I've tried the following:
SELECT MIN(x), MIN(y), MAX(x), MAX(y)
FROM nodeCartesian
WHERE id IN
(
SELECT nodeid
FROM waypoint
WHERE wayid IN
(
SELECT id
FROM way
WHERE closed = 1
)
);
But this gets the minimum of all nodes since they don't get properly grouped together into their respective 'areas'.
P.S. I am dealing with a 2D r-tree, so I know what I wrote isn't correct, but I just wrote what I'm having difficulty with. Also, I tried researching how to apply inner joins to my statement, but couldn't figure out how :(, so if you think that may help my performance as well, I would love to hear it. Another thing is that query1 deals with 2+ million rows, while query2 deals with approximately 340,000 rows, and I estimated that it will take about 1 day for query2 to finish.
Thanks
I am not sure about your schema; however, I think that something like this by including a group by your area should do it
SELECT m.areaid, MIN(n.x), MIN(n.y), MAX(n.x), MAX(n.y)
FROM
nodeCartesian n
INNER JOIN waypoint wp ON n.id = wp.nodeid
INNER JOIN way w ON wp.wayid = w.id
INNER JOIN middleTable m ON n.id = m.nodeid
WHERE
w.closed = 1
GROUP BY
m.areaid
Note: calling a SELECT query multiple times in a loop is a bad idea, because each call has a great overhead which makes it really slow. Making a single query returning all the relevant rows and then looping through them in code is much faster.
I have a quick question about the following piece of code. Why can we use 'NA' = for the subquery ? I mean, the subquery might return a group of values, not a single one, right? Could anyone tell me the reason? Many thanks for your time and attention.
proc sql;
select lastname, first name
from sasuser.staffmaster
where 'NA' =
(select jobcategory
from sasuser.supervisors
where staffmaster.empid = supervisors.empid);
quit;
Thanks again.
Assuming EMPID is a unique ID for an employee (I hope it is?), and each employee has only one supervisor, that query should resolve to a single row every time. (A single row for each row returned from the outer query, of course, which is important. Think of it like a join - that's basically what that is, a slightly oddly phrased join, which often will be turned into an actual join by the SQL parser.)
In general, however, sure, it could resolve to multiple rows. SAS will let you do the query, and if it returns just one row it works; if it returns 2+ rows, it fails. As Quentin pointed out in comments, this is a correlated subquery.
I have two tables I would like to call, but I am not sure if it is possible to combine them into one query or I have to some how call 2 different queries.
Basically I have 2 tables:
1) item_table: name/id etc. + category ID
2) category_table: categoryID, categoryName, categoryParentID.
The parent categories are also inside the same table with their own name.
I would like to call on my details from item_table, as well as getting the name of the category, as well as the NAME of the parent category.
I know how to get the item_table data, plus the categoryName through an INNER JOIN. But can I use the same query to get the categoryParent's name?
If not, what would be the mist efficient way to do it? The rest of the code is in C++.
SELECT item_table.item_name, c1.name AS CatName, c2.name AS ParentCatName
FROM item_table join category_table c1 on item_table.categoryID=c1.categoryID
LEFT OUTER JOIN category_table c2 ON c2.categoryID = c1.categoryParentID
SQL Fiddle: here