My data:
id value
1 10
1 20
1 60
2 10
3 10
3 30
How to compute column 'change'?
id value change | my comment, how to compute
1 10 10 | 20-10
1 20 40 | 60-20
1 60 40 | default_value-60. In this example default_value=100
2 10 90 | default_value-10
3 10 20 | 30-10
3 30 70 | default_value-30
In other words: if row of id is last, then compute 100-value,
else compute next_value-value_now
You can access the value of the "next" (or "previous") row using a window function. The concept of a "next" row only makes sense if you have a column to define an order on the rows. You said you have a date column on which you can order the result. I used the column name your_date_column for this. You need to replace that with the actual column name of course.
select id,
value,
lead(value, 1, 100) over (partition by id order by your_date_column) - value as change
from the_table
order by id, your_date_column
lead(value, 1, 100) says: take the column value of the "next" row (that's the 1). If there is no such row, use the default value 100 instead.
Join on a subquery and use ROW_NUMBER to find the last value per group
WITH CTE AS(
SELECT id,value,
ROW_NUMBER() OVER (PARTITION BY id ORDER BY date) rn,
(LEAD(value) OVER (PARTITION BY id ORDER BY date)-value) change FROM t)
SELECT cte.id,cte.value,
(CASE WHEN cte.change IS NULL THEN 100-cte.value ELSE cte.change END)as change FROM cte LEFT JOIN
(SELECT id,MAX(rn) mrn FROM cte
GROUP BY id) as x
ON x.mrn=cte.rn AND cte.id=x.id
FIDDLE
Related
I have a table containing data that has a column named id that looks like below:
id
value 1
value 2
value 3
1
244
550
1000
1
251
551
700
1
540
60
1200
...
...
...
...
2
19
744
2000
2
10
903
100
2
44
231
600
2
120
910
1100
...
...
...
...
I want to take 50 sample rows per id that exists but if less than 50 exist for the group to simply take the entire set of data points.
For example I would like a maximum 50 data points randomly selected from id = 1, id = 2 etc...
I cannot find any previous questions similar to this but have tried taking a stab at at least logically working through the solution where I could iterate and union all queries by id and limit to 50:
SELECT * FROM (SELECT * FROM schema.table AS tbl WHERE tbl.id = X LIMIT 50) UNION ALL;
But it's obvious that you cannot use this type of solution because UNION ALL requires aggregating outputs from one id to the next and I do not have a list of id values to use in place of X in tbl.id = X.
Is there a way to accomplish this by gathering that list of unique id values and union all results or is there a more optimal way this could be done?
If you want to select a random sample for each id, then you need to randomize the rows somehow. Here is a way to do it:
select * from (
select *, row_number() over (partition by id order by random()) as u
from schema.table
) as a
where u <= 50;
Example (limiting to 3, and some row number for each id so you can see the selection randomness):
setup
DROP TABLE IF EXISTS foo;
CREATE TABLE foo
(
id int,
value1 int,
idrow int
);
INSERT INTO foo
select 1 as id, (1000*random())::int as value1, generate_series(1, 100) as idrow
union all
select 2 as id, (1000*random())::int as value1, generate_series(1, 100) as idrow
union all
select 3 as id, (1000*random())::int as value1, generate_series(1, 100) as idrow;
Selection
select * from (
select *, row_number() over (partition by id order by random()) as u
from foo
) as a
where u <= 3;
Output:
id
value1
idrow
u
1
542
6
1
1
24
86
2
1
155
74
3
2
505
95
1
2
100
46
2
2
422
33
3
3
966
88
1
3
747
89
2
3
664
19
3
In case you are looking to get 50 (or less) from each group of IDs then you can use windowing -
From question - "I want to take 50 sample rows per id that exists but if less than 50 exist for the group to simply take the entire set of data points."
Query -
with data as (
select row_number() over (partition by id order by random()) rn,
* from table_name)
select * from data where rn<=50 order by id;
Fiddle.
Your description of trying to get the UNION ALL without specifying all the branches ahead of time is aiming for a LATERAL join. And that is one way to solve the problem. But unless you have a table of all distinct ids, you would have to compute one on the fly. For example (using the same fiddle as Pankaj used):
with uniq as (select distinct id from test)
select foo.* from uniq cross join lateral
(select * from test where test.id=uniq.id order by random() limit 3) foo
This could be either slower or faster than the Window Function method, depending on your system and your data and your indexes. In my hands, it was quite a bit faster even with the need to dynamically compute the list of distinct ids.
I am new to databases and postgres as such.
I have a table called names which has 2 columns name and value which gets updated every x seconds with new name value pairs. My requirement is to retain only 3 positive and 3 negative values at any point of time and delete the rest of the rows during each table update.
I use the following query to delete the old rows and retain the 3 positive and 3 negative values ordered by value.
delete from names
using (select *,
row_number() over (partition by value > 0, value < 0 order by value desc) as rn
from names ) w
where w.rn >=3
I am skeptical to use a conditional like value > 0 in a partition statement. Is this approach correct?
For example,
A table like this prior to delete :
name | value
--------------
test | 10
test1 | 11
test1 | 12
test1 | 13
test4 | -1
test4 | -2
My table after delete should look like :
name | value
--------------
test1 | 13
test1 | 12
test1 | 11
test4 | -1
test4 | -2
demo:db<>fiddle
This works generally as expected: value > 0 clusters the values into all numbers > 0 and all numbers <= 0. The ORDER BY value orders these two groups as expected well.
So, the only thing, I would change:
row_number() over (partition by value >= 0 order by value desc)
remove: , value < 0 (Because: Why should you group the positive values into negative and other? You don't have any negative numbers in your positive group and vice versa.)
change: value > 0 to value >= 0 to ignore the 0 as long as possible
For deleting: If you want to keep the top 3 values of each direction:
you should change w.rn >= 3 into w.rn > 3 (it keeps the 3rd element as well)
you need to connect the subquery with the table records. In real cases you should use id columns for that. In your example you could take the value column: where n.value = w.value AND w.rn > 3
So, finally:
delete from names n
using (select *,
row_number() over (partition by value >= 0 order by value desc) as rn
from names ) w
where n.value = w.value AND w.rn > 3
If it's not a hard requirement to delete the other rows, you could instead select only the rows you're interested in:
WITH largest AS (
SELECT name, value
FROM names
ORDER BY value DESC
LIMIT 3),
smallest AS (
SELECT name, value
FROM names
ORDER BY value ASC
LIMIT 3)
SELECT * FROM largest
UNION
SELECT * FROM smallest
ORDER BY value DESC
I have a column in the following format:
Time Value
17:27 2
17:27 3
I want to get the distinct rows based on one column: Time. So my expected result would be one result. Either 17:27 3 or 17:27 3.
Distinct
T-SQL uses distinct on multiple columns instead of one. Distinct would return two rows since the combinations of Time and Value are unique (see below).
select distinct [Time], * from SAPQMDATA
would return
Time Value
17:27 2
17:27 3
instead of
Time Value
17:27 2
Group by
Also group by does not appear to work
select * from table group by [Time]
Will result in:
Column 'Value' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.
Questions
How can I select all unique 'Time' columns without taking into account other columns provided in a select query?
How can I remove duplicate entries?
This is where ROW_NUMBER will be your best friend. Using this as your sample data...
time value
-------------------- -----------
17:27 2
17:27 3
11:36 9
15:14 5
15:14 6
.. below are two solutions with that you can copy/paste/run.
DECLARE #youtable TABLE ([time] VARCHAR(20), [value] INT);
INSERT #youtable VALUES ('17:27',2),('17:27',3),('11:36',9),('15:14',5),('15:14',6);
-- The most elegant way solve this
SELECT TOP (1) WITH TIES t.[time], t.[value]
FROM #youtable AS t
ORDER BY ROW_NUMBER() OVER (PARTITION BY t.[time] ORDER BY (SELECT NULL));
-- A more efficient way solve this
SELECT t.[time], t.[value]
FROM
(
SELECT t.[time], t.[value], ROW_NUMBER() OVER (PARTITION BY t.[time] ORDER BY (SELECT NULL)) AS RN
FROM #youtable AS t
) AS t
WHERE t.RN = 1;
Each returns:
time value
-------------------- -----------
11:36 9
15:14 5
17:27 2
Id values
1 10
1 20
1 30
1 40
2 3
2 9
2 0
3 14
3 5
3 7
Answer should be
Id values
1 30
2 3
3 7
I tried as below
Select distinct
id,
(select max(values)
from table
where values not in(select ma(values) from table)
)
You need the row_number window function. This adds a column with a row count for each group (in your case the ids). In a subquery you are able to ask for the second row of each group.
demo:db<>fiddle
SELECT
id, values
FROM (
SELECT
*,
row_number() OVER (PARTITION BY id ORDER BY values DESC)
FROM
table
) s
WHERE row_number = 2
I have a result set from which I want to get next n rows (or previous n rows) after (before) the row that matches a particular cell value.
So for example, here is my data:
A B C
1 10 100
2 20 200
3 30 300
4 40 400
5 50 500
6 60 600
I am interested to get next 3 rows after the row where C=300, including C=300 row, so my output should be
A B C
3 30 300
4 40 400
5 50 500
6 60 600
With FETCH and OFFSET, you need to know the exact position number of the row, here I have to search where the data condition, i.e C=300 resides so I cannot assume that it will be the 3rd row.
select *
from table
order by C asc
Assuming you've got a table named sample, you could use a nested query and window functions to solve your issue, something like:
select *
from (
select *, lag(c,3) over(order by c asc) as three_back
from sample
where sample.c >= 300
) t
where coalesce(three_back,300) = 300
If your rows are ordered by the column value you are interested in then
SELECT *
FROM table_name
WHERE column_name >= x
ORDER BY column_name
LIMIT n
should do it. If not you’ll have to get creative
If your column values are unique and you want to order by another value then
SELECT *
FROM table_name
WHERE other_column >= (
SELECT other_column
FROM table_name
WHERE column_value = x
)
ORDER BY other_column
LIMIT n
If your column values are not unique you can
SELECT MIN(other_column)
in the inner select. This finds the first occurrence (using the other column to order by), and then retrieves the next (n - 1) rows