I am new to sql side. so if this questoin sound very easy then please spare me. I have a 4 coloumns in a sql table.Let say A,B,C,D . For any BC combination I may get any number of rows. I need to get at max 3 rows (which inturn give me 3 unique value of A for that BC ombination) for these selected rows i should have Top 3 Max value of D. As compare to other entries for that BC combination.
So there can be any number of BC combination so the above logic should imply to all of them.
Most databases support ranking functions. With these, you can do what you want as follows:
select A, B, C, D
from (select t.*,
row_number() over (partition by B, C order by D desc) as seqnum
from t
) t
where seqnum <= 3
order by B, C, D desc
The row_number() function creates a sequencial number. This number starts at "1" in very B,C group, and is ordered by the value of D descending.
Related
I have a table in Postgres with timestamp and 6 columns (A,B,C,D,E,F) with values. Every 10 minutes new record is appended to this table, however for columns B, D, F actual value is fetched only every 30 minutes meaning that only every 3rd row is not null.
I would like to write a query that outputs most recent record per every column. The only thing that comes to my mind is to write 2 queries:
SELECT A,C,E
FROM data_prices
ORDER BY date_of_record DESC LIMIT 1
SELECT B,D,F
FROM data_prices
WHERE B is not null, D is not null, F is not null
ORDER BY date_of_record DESC LIMIT 1
And then join the results into 1 table with 6 columns and 1 row. I don't know, however how to do that because in the documentation I found operations like UNION, INTERSECT, EXCEPT which append data rather than creating one wider table. Any ideas how to join these 2 selects into 1 table with 6 columns? Or maybe smarter way to get latest non NULL result per column in table?
Unfortunately, Postgres does not support lag(ignore nulls).
One method uses first_value():
select date_of_record, a,
first_value(b) over (order by (b is not null) desc, date_of_record desc) as last_b,
c,
first_value(d) over (order by (d is not null) desc, date_of_record desc) as last_d,
e,
first_value(f) over (order by (f is not null) desc, date_of_record desc) as last_f
from t
order by date_of_record desc
limit 1;
Say I have a table with columns A, B, C, and D.
What I want is to
get all the distinct combinations of A and B that there exists in the original table.
for every such combination, I want extract a SINGLE row that has that combination, plus I also want its C and D value.
there will probably be multiple rows that have that particular combination of A and B. In that case, I still only want just one row, and it should be the one with the highest value in the C column.
For example, if in my original table I have A = Male or Female, B = Tall or Short, and C = Age, and D is something else, then I want to end up with a table with 4 rows, each having one of these combinations:
Male, Tall, …, …
Female Tall, …, ...
Male Short, …, ...
Female Short, …, …
where each row should belong to the person with the biggest age, and then their respective D value as well.
Gordon's answer is wrong. Use this:
select a, b, c, d
from (select t.*,
row_number() over (partition by a, b order by c) as seqnum
from t
) t
where seqnum = 1;
if you are trying to group by columns A and B, and get max,sum,avg or any other function value of columns C and D a simple group by clause might work, example;
select a, b, max(c), max(d)
from table
group by a,b
You can use row_number():
select a, b, c, d
from (select t.*,
row_number() over (partition by a, b order by c desc) as seqnum
from t
) t
where seqnum = 1;
A B C D
1pm a 1 1
2pm a 2 2
3pm b 1 1
4pm b 2 2
5pm a 3 1
6pm a 4 2
When I do row_number() over (partition by B order by A) as C ., I get the column C. How do I get column D?
You need to assign a group to the "adjacent" values. One simple method is the difference of row numbers:
select a, b,
row_number() over (partition by b, (seqnum_a - seqnum_ab) order by a) as d
from (select t.*,
row_number() over (order by a) as seqnum_a,
row_number() over (partition by b order by a) as seqnum_ab
from t
) t;
The difference of row numbers is one solution to some types of gaps-and-islands problems (basically what you are asking for). Why it works is a little tricky to explain. I find that if someone sees the results of the subquery, they will usually get why the difference identifies the adjacent rows.
I have a PostgreSQL table example with three columns: a INT, b INT, c TEXT.
For each value of a I want the c with the highest value of b, together with the sum of all b. Something like (if there was an ARGMAX function):
SELECT a, ARGMAX(c for MAX(b)), SUM(b) FROM example GROUP BY a
I've found a lot of solutions with varying techniques to get the ARGMAX bit, but none of them seem to use GROUP BY, so I was wondering what we most efficient way would be to capture the SUM (or other aggregate functions) as well.
This can be easily achieved using window functions:
SELECT a, b, c, s
FROM (
SELECT a, b, c,
ROW_NUMBER() OVER (PARTITION BY a ORDER BY b DESC) AS rn,
SUM(b) OVER (PARTITION BY a) AS s
FROM example) AS t
WHERE t.rn = 1
ROW_NUMBER enumerates records within each a partition: the record having the highest b value is assigned a value of 1, next record a value of 2, etc.
SUM(b) OVER (PARTITION BY a) returns the sum of all b within each a partition.
Suppose I have a table filled with the data below, what SQL function or query I should use in db2 to retrieve all rows having the FIRST field FLD_A with value A, the FIRST field FLD_A with value B..and so on?
ID FLD_A FLD_B
1 A 10
2 A 20
3 A 30
4 B 10
5 A 20
6 C 30
I am expecting a table like below; I am aware of grouping done by function GROUP BY but how can I limit the query to return the very first of each group?
Essentially I would like to have the information about the very first row where a new value for FLD_A is appearing for the first time?
ID FLD_A FLD_B
1 A 10
4 B 10
6 C 30
Try this it works in sql
SELECT * FROM Table1
WHERE ID IN (SELECT MIN(ID) FROM Table1 GROUP BY FLD_A)
A good way to approach this problem is with window functions and row_number() in particular:
select t.*
from (select t.*,
row_number() over (partition by fld_a order by id) as seqnum
from table1
) t
where seqnum = 1;
(This is assuming that "first" means "minimum id".)
If you use t.*, this will add one extra column to the output. You can just list the columns you want to avoid this.