I have the following table in PostgreSQL DB:
DB exempt
I need a PostgreSQL command to get a specific value from tbl column, based on time_launched and id columns. More precisely, I need to get a value from tbl column which corresponds to a specific id and latest (time-wise) value from time_launched column. Consequently, the request should return "x" as an output.
I've tried those requests (using psycopg2 module) but they did not work:
db_object.execute("SELECT * FROM check_ids WHERE id = %s AND MIN(time_launched)", (id_variable,))
db_object.execute(SELECT DISTINCT on(id, check_id) id, check_id, time_launched, tbl, tbl_1 FROM check_ids order by id, check_id time_launched desc)
Looks like a simple ORDER BY with a LIMIT 1 should do the trick:
SELECT tbl
FROM check_ids
WHERE id = %s
ORDER BY time_launched DESC
LIMIT 1
The WHERE clause filters results by the provided id, the ORDER BY clause ensures results are sorted in reverse chronological order, and LIMIT 1 only returns the first (most recent) row
Related
I'm trying to count the number of unique pool operators for every permit # in a table but am having trouble putting this value in a new column dedicated to that count.
So I have 2 tables: doh_analysis; doh_pools.
Both of these tables have a "permit" column (TEXT), but doh_analysis has about 1000 rows with duplicates in the permit column but occasional unique values in the operator column (TEXT).
I'm trying to fill a column "operator_count" in the table "doh_pools" with a count of unique values in "pooloperator" for each permit #.
So I tried the following code but am getting a syntax error at or near "(":
update doh_pools
set operator_count = select count(distinct doh_analysis.pooloperator)
from doh_analysis
where doh_analysis.permit ilike doh_pools.permit;
When I remove the "select" from before the "count" I get "SQL Error [42803]: ERROR: aggregate functions are not allowed in UPDATE".
I can successfully query a list of distinct permit-pooloperator pairs using:
select distinct permit, pooloperator
from doh_analysis;
And I can query the # of unique pooloperators per permit 1 at a time using:
select count(distinct pooloperator)
from doh_analysis
where permit ilike '52-60-03054';
But I'm struggling to insert a count of unique pairs for each permit # in the operatorcount column.
Is there a way to do this?
There is certainly a better way of doing this but I accomplished my goal by creating 2 intermediary tables and the updating the target table with values from the 2nd intermediate table like so:
select distinct permit, pooloperator
into doh_pairs
from doh_analysis;
select permit, count(distinct pooloperator)
into doh_temp
from doh_pairs
group by permit;
select count(distinct permit)
from doh_temp;
update doh_pools
set operator_count = doh_temp.count
from doh_temp
where doh_pools.permit ilike doh_temp.permit
and doh_pools.permit is not NULL
returning count;
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;
Is there any way to get rowid of a record in postgres??
In oracle i can use like
SELECT MAX(BILLS.ROWID) FROM BILLS
Yes, there is ctid column which is equivalent for rowid. But is useless for you. Rowid and ctid are physical row/tuple identifiers => can change after rebuild/vacuum.
See: Chapter 5. Data Definition > 5.4. System Columns
The PostgreSQL row_number() window function can be used for most purposes where you would use rowid. Whereas in Oracle the rowid is an intrinsic numbering of the result data rows, in Postgres row_number() computes a numbering within a logical ordering of the returned data. Normally if you want to number the rows, it means you expect them in a particular order, so you would specify which column(s) to order the rows when numbering them:
select client_name, row_number() over (order by date) from bills;
If you just want the rows numbered arbitrarily you can leave the over clause empty:
select client_name, row_number() over () from bills;
If you want to calculate an aggregate over the row number you'll have to use a subquery:
select max(rownum) from (
select row_number() over () as rownum from bills
) r;
If all you need is the last item from a table, and you have a column to sort sequentially, there's a simpler approach than using row_number(). Just reverse the sort order and select the first item:
select * from bills
order by date desc limit 1;
Use a Sequence. You can choose 4 or 8 byte values.
http://www.neilconway.org/docs/sequences/
Add any unique column to your table(name maybe rowid).
And prevent changing it by creating BEFORE UPDATE trigger, which will raise exception if someone will try to update.
You may populate this column with sequence as #JohnMudd mentioned.
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
The following code works and creates a temporary table with a sequence number which is restarted for every new name:
with results as (select row_number() over (partition by name order BY name) as mytid,name from telephn_table)
select * from results order by name
My objective however is to insert the new sequence number permanently into the telephone table.
How do I transfer the new sequence number from the results table to the telephone table? I have come across the following for MySql but was not able to convert it to Postgresql.
MySQL: Add sequence column based on another field
Can anyone help?
If memory serves, row_number() returns the number within its own partition. In other words, row_number() over (partition by name order BY name) would return 1 for each row except duplicates. You likely want rank() over (order by name) instead.
After a long discussion:
update telephn_table
set sid = rows.new_sid
from (select pkey,
row_number() over (partition BY name) as new_sid,
name
from telephn_table
) as rows
where rows.pkey = telephn_table.pkey;
THIS WORKS! (See my OP link to a previous MySql link. In Postgresql it works without need for a temporary table)
alter table telephn_table add column tid integer default 0;
UPDATE telephn_table set tid=(SELECT count(*)+1 from telephn_table t where t.sid < telephn_table.sid and telephn_table.name=t.name)