Getting group by attribute in nested query - postgresql

I am trying to find the most frequent value in a postgresql table. The problem is that I also want to "group by" in that table and only get the most frequent from the values that have the same name.
So I have the following query:
select name,
(SELECT value FROM table where name=name GROUP BY value ORDER BY COUNT(*) DESC limit 1)
as mfq from table group by name;
So, I am using where name=name, trying to get the outside group by attribute "name", but it doesn't seem to work. Any ideas on how to do it?
Edit: for example in the following table:
name value
a 3
a 3
a 3
b 2
b 2
I want to get:
name value
a 3
b 2
but the above statement gives:
name value
a 3
b 3
instead, since where doesn't work correctly.

There is a dedicated function in PostgreSQL for this case: the mode() ordered-set aggregate:
select name, mode() within group (order by value) mode_value
from table
group by name;
which returns the most frequent input value (arbitrarily choosing the first one if there are multiple equally-frequent results) -- which is the same behavior as with your order by count(*) desc limit 1.
It is available from PostgreSQL 9.4+.
http://rextester.com/GHGJH15037

If you want your query to work, you need table aliases. Table aliases and qualified column names are always a good idea:
select t.name,
(select t2.value
from table t2
where t2.name = t.name
group by t2.value
order by COUNT(*) desc
limit 1
) as mfq
from table t
group by t.name;

Related

postgresql selecting the most representative value

I have a table in which objects have ids and they have names. The ids are correct by definition, the names are almost always correct, but sometimes dirty incoming data causes names to be null or even wrong.
So I do a query like
SELECT id, name, AGGR1(a) as a, AGGR2(b) as b, AGGR3(c) as c
FROM my_table
WHERE d = 3
GROUP BY id
I'd like to have name in the results, but of course the above is wrong. I'd have to group on id, name, in which case what should be one row sometimes becomes more than one -- say, id 2 has names 'John' (correct), 'Jon' (no, but only 1%), or NULL (also a small fraction).
Is there a construct or idiom in postgresql that lets me select what a human looking at the list would say is obviously the consensus name?
(I hear our postgres installation is finally being upgraded soon, if that matters here.)
sample output, in case prose wasn't clear
SELECT id, name, COUNT(id) as c
FROM my_table
WHERE d = 3
GROUP BY id
id name c
2 John 2000
2 Jon 3
2 (NULL) 5
vs
id name c
2 John 2008
You can get the names with
WITH names as (
SELECT
id,
name,
ROW_NUMBER() OVER (PARTITION BY id ORDER BY COUNT(1) DESC) as rn
FROM my_table
GROUP BY id, name
)
SELECT id, name
FROM names
WHERE rn=1;
and then do your calculations by id only, joining names from this query.

Postgres query filter by non column in table

i have a challenge whose consist in filter a query not with a value that is not present in a table but a value that is retrieved by a function.
let's consider a table that contains all sales on database
id, description, category, price, col1 , ..... col n
i have function that retrieve me a table of similar sales from one (based on rules and business logic) . This function performs a query again on all records in the sales table and match validation in some fields.
similar_sales (sale_id integer) - > returns a integer[]
now i need to list all similar sales for each one present in sales table.
select s.id, similar_sales (s.id)
from sales s
but the similar_sales can be null and i am interested only return sales which contains at least one.
select id, similar
from (
select s.id, similar_sales (s.id) as similar
from sales s
) q
where #similar > 1 (Pseudocode)
limit x
i can't do the limit in subquery because i don't know what sales have similar or not.
I just wanted do a subquery for a set of small rows and not all entire table to get query performance gains (pagination strategy)
you can try this :
select id, similar
from sales s
cross join lateral similar_sales (s.id) as similar
where not isempty(similar)
limit x

selecting a distinct column with alias table not working in postgres

SELECT pl_id,
distinct ON (store.store_ID),
in_user_id
FROM plan1.plan_copy_levl copy1
INNER JOIN plan1._PLAN_STORE store
ON copy1.PLAN_ID = store .PLAN_ID;
while running this query in postgres server i am getting the below error..How to use the distinct clause..in above code plan 1 is the schema name.
ERROR: syntax error at or near "distinct" LINE 2: distinct ON
(store.store_ID),
You are missing an order by where the first set of rows should be the ones specified in the distinct on clause. Also, the distinct on clause should be at start of the selection list.
Try this:
SELECT distinct ON (store_ID) store.store_ID, pl_id,
in_user_id
FROM plan1.plan_copy_levl copy1
INNER JOIN plan1._PLAN_STORE store
ON copy1.PLAN_ID = store .PLAN_ID
order by store_ID, pl_id;

Query Oracle SqlDeveloper

I want to write a SELECT statement to create a list of jobs containing the job_title and the mean value of the two fields MIN_SALARY and MAX_SALARY sorted from the highest mean value to the lowest.
I wrote this:
select job_title, MAX_SALARY, MIN_SALARY, count(*)
from HR.EMPLOYEES, HR.JOBS
order by count(*) desc
group by j.job_title;
but it gives error:
ORA-00933: SQL command not properly ended 00933. 00000 - "SQL command not properly ended"
I think that you should have this query
select job_title, MAX_SALARY, MIN_SALARY, count(*)
from HR.EMPLOYEES, HR.JOBS
group by job_title
order by count(*) desc
I have reordered the clauses and removed the spurious j
See - select

how to get rowNum like column in sqlite IPHONE

I have an Sqlite database table like this (with out ascending)
But i need to retrive the table in Ascending order by name, when i set it ascending order the rowId changes as follows in jumbled order
But i need to retrieve some limited number of contacts 5 in ascending order every time
like Aaa - Eeee and then Ffff- Jjjjj ......
but to se**t limits like 0-5 5-10 .... ** it can able using rowids since they are in jumble order
So i need another column like (rowNum in oracle) wich is in order 1234567... every time as follows
how to retrive that column with existing columns
Note: WE DONTE HAVE ROWNUM LIKE COLUMN IN SQLITE
The fake rownum solution is clever, but I am afraid it doesn't scale well (for complex query you have to join and count on each row the number of row before current row).
I would consider using create table tmp as select /*your query*/.
because in the case of a create as select operation the rowid created when inserting
the rows is exactly what would be the rownum (a counter). It is specified by the SQLite doc.
Once the initial query has been inserted, you only need to query the tmp table:
select rowid, /* your columns */ from tmp
order by rowid
You can use offset/limit.
Get the first, 2nd, and 3rd groups of five rows:
select rowid, name from contactinfo order by name limit 0, 5
select rowid, name from contactinfo order by name limit 5, 5
select rowid, name from contactinfo order by name limit 10, 5
Warning, using the above syntax requires SQLite to read through all prior records in sorted order. So to get the 10th record for statement number 3 above SQLite needs to read the first 9 records. If you have a large number of records this can be problematic from a performance standpoint.
More info on limit/ offset:
Sqlite Query Optimization (using Limit and Offset)
Sqlite LIMIT / OFFSET query
This is a way of faking a RowNum, hope it helps:
SELECT
(SELECT COUNT(*)
FROM Names AS t2
WHERE t2.name < t1.name
) + (
SELECT COUNT(*)
FROM Names AS t3
WHERE t3.name = t1.name AND t3.id < t1.id
) AS rowNum,
id,
name
FROM Names t1
ORDER BY t1.name ASC
SQL Fiddle example