I want to calculate the difference between the previous and the current column and make it a new column named increase. For this, I'm using the lag window function. The value of the first column is not defined since no previous column exists. I know that a 3rd parameter specifies the default value. However, it depends. For the first row, I want to use the value of another column e.g. the one of count from that current row. This assumes that 0 is increased to count for the first row which is what I need. Specifying the column name as 3rd argument for the lag function does not work correctly and neither does using 0. How can it be done? I'm getting strange results such as quite a random result or even negative numbers.
SELECT *, mycount - lag(mycount, 1) OVER (ORDER BY id, messtime ASC) AS increase FROM measurements;
Window functions cannot be nested either:
ERROR: window function calls cannot be nested
There is another issue with your query: So far your results are in random order, so you may think you are seeing problems that don't exist.
Add ORDER BY id, messtime to your query to see the rows in order. Now you can compare one row with its predecessor directly. Are there still issues? If so, which exactly?
SELECT *, "count" - lag("count", 1) OVER (ORDER BY id, messtime) AS increase
FROM measurements
ORDER BY id, messtime;
COUNT is a reserved word in SQL. It seems the DBMS thinks you want to nest COUNT and LAG somehow.
Use another column name or use quotes for the column:
SELECT *, "count" - lag("count", 1) OVER
Related
I am trying to add a column to a collection by multiplying the 0.9 to existing database column recycling. but I get a run time error.
I tried to multiply 0.9 direction in the function but it is showing error, so I created the class and multiplied it there yet no use. what could be the problem?
Your error message is telling you what the problem is: your database query is using GROUP BY in an invalid way.
It doesn't make sense to group by one column and then select other columns (you've selected all columns in your case); what values would they contain, since you haven't grouped by them as well (and get one row returned per group)? You either have to group by all the columns you're selecting for, and/or use aggregates such as SUM for the non-grouped columns.
Perhaps you meant to ORDER BY that column (orderBy(dt.recycling.asc()) if ascending order in QueryDSL format), or to select all rows with a particular value of that column (where(dt.recycling.eq(55)) for example)?
I am trying to understand how the ORDER BY clause in the OVER() window function is different from ORDER BY clause in generic SQL.
I was solving the following problem: https://www.pgexercises.com/questions/aggregates/nummembers.html
Produce a monotonically increasing numbered list of members (including guests),
ordered by their date of joining. Remember that member IDs are not guaranteed to be sequential.
The following query is one of the accepted solutions:
SELECT COUNT(*) OVER(ORDER by joindate), firstname, surname FROM cd.members;
As per my understanding, since we are not supplying a PARTITION BY clause in the OVER() function, all the rows in cd.members table form one big partition (let's call it X). When the window function runs, it should order X by joindate, and then COUNT(*) on X would return the number of rows in X which is just the number of rows in cd.members.
But this understanding is incorrect. The 'Answers and Discussion' accompanying the aforementioned problem states:
Since we define an order for the window function, for any given row the window is: start of the dataset -> current row.
The PG documentation on window function states:
You can also control the order in which rows are processed by window functions using ORDER BY within OVER. (The window ORDER BY does not even have to match the order in which the rows are output.)
What I cannot comprehend is why will ORDER BY inside the OVER() stop at the current row? Could you please elaborate how this is working?
Thank you for reading through.
I don't know what to add beyond what the docs (same page as you already linked to) already say:
By default, if ORDER BY is supplied then the frame consists of all
rows from the start of the partition up through the current row, plus
any following rows that are equal to the current row according to the
ORDER BY clause.
I don't now if this required by the SQL standard, but it certainly seems reasonable. Why specify an ORDER BY if you expect it to have no observable effect?
I am new to tableau.
Just recently, I have encountered a problem regarding getting a string value from the previous row
I tried using Previous_Value function but it does not work. :<
Image of the error
Instead of previous value which could be tricky when dealing with partitions, you may use the LookUp function
Returns the value of the expression in a target row, specified as a relative offset from the current row. Use FIRST() +n and LAST() -n as part of your offset definition for a target relative to the first/last rows in the partition. If offset is omitted, the row to compare to can be set on the field menu. This function returns NULL if the target row cannot be determined.
You can create a calculated field Order ID Lookup - 1:
LOOKUP(max([Order ID]),-1)
Remember that Lookup requires an aggregate value as first argoment (in my example you can use either min, max, etc...) followed by the offset you need (in this example it's 1 for previous record).
Once you have your previous value, you can create another calculated field Check:
if max([Order ID]) = [Order ID Lookup - 1]
then '='
else '!='
end
Since Lookup needs an aggregate function, you should "wrap" your Order ID with an aggregation function as well in order to compare those 2 values.
Here's the final result:
From every references that I search how to do cumulative sum / running total. they said it's better using windows function, so I did
select grandtotal,sum(grandtotal)over(order by agentname) from call
but I realize that the results are okay as long as the value of each rows are different. Here is the result :
Is There anyway to fix this?
You might want to review the documentation on window specifications (which is here). The default is "range between" which defines the range by the values in the row. You want "rows between":
select grandtotal,
sum(grandtotal) over (order by agentname rows between unbounded preceding and current row)
from call;
Alternatively, you could include an id column in the sort to guarantee uniqueness and not have to deal with the issue of equal key values.
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.