How to reference a column in the select clause in the order clause in SQLAlchemy like you do in Postgres instead of repeating the expression twice - postgresql

In Postgres if one of your columns is a big complicated expression you can just say ORDER BY 3 DESC where 3 is the order of the column where the complicated expression is. Is there anywhere to do this in SQLAlchemy?


As Gord Thompson observes in this comment, you can pass the column index as a text object to group_by or order_by:
q = sa.select(sa.func.count(), tbl.c.user_id).group_by(sa.text('2')).order_by(sa.text('2'))
serialises to
SELECT count(*) AS count_1, posts.user_id
FROM posts GROUP BY 2 ORDER BY 2
There are other techniques that don't require re-typing the expression.
You could use the selected_columns property:
q = sa.select(tbl.c.col1, tbl.c.col2, tbl.c.col3)
q = q.order_by(q.selected_columns[2]) # order by col3
You could also order by a label (but this will affect the names of result columns):
q = sa.select(tbl.c.col1, tbl.c.col2, tbl.c.col3.label('c').order_by('c')

Related

Subquery with `WHERE` on function calls with outer query grouped by the function calls gives "subquery uses ungrouped column from outer query"

Consider this situation where age_group(.) is a function that returns an age bracket for an age (0-17: 'minor', 18-64: 'adult' etc.)
SELECT
date_of_data,
age_group(age),
count(1),
(SELECT avg(salary)
FROM tbl2
WHERE age_group(tbl2.age) = age_group(tbl1.age)
AND tbl2.date_of_file = tbl1.date_of_file
AND type = 'junior') AS average_salary_as_junior,
(SELECT avg(salary)
FROM tbl2
WHERE age_group(tbl2.age) = age_group(tbl1.age)
AND tbl2.date_of_file = tbl1.date_of_file
AND type = 'senior') AS average_salary_as_senior,
(SELECT avg(salary)
FROM tbl2
WHERE age_group(tbl2.age) = age_group(tbl1.age)
AND tbl2.date_of_file = tbl1.date_of_file
AND type = 'principal') AS average_salary_as_principal,
-- 15 more types to go
FROM tbl1
GROUP BY
date_of_data, age_group(age);
This will not work unless the outer query is grouped by age in contrast to age_group(age), because the subquery uses age as an argument to a function, despite being the same function:
subquery uses ungrouped column "tbl1.age" from outer query..
If I group by age instead of age_group(age), there will be redundant identical records in the output.
Conditional aggregation might be a solution, and so is using DISTINCT on the whole output, albeit inefficient. Not sure if there are more techniques to achieve the same, but I am wondering whether there's a way to make Postgres realise that the same function call exists in the GROUP BY clause, and permit such a query to execute.

oracle: grouping on merged columns

I have a 2 tables FIRST
id,rl_no,adm_date,fees
1,123456,14-11-10,100
2,987654,10-11-12,30
3,4343,14-11-17,20
and SECOND
id,rollno,fare,type
1,123456,20,bs
5,634452,1000,bs
3,123456,900,bs
4,123456,700,bs
My requirement is twofold,
1, i first need to get all columns from both tables with common rl_no. So i used:
SELECT a.ID,a.rl_no,a.adm_date,a.fees,b.rollno,b.fare,b.type FROM FIRST a
INNER JOIN
SECOND b ON a.rl_no = b.rollno
The output is like this:
id,rl_no,adm_date,fees,rollno,fare,type
1,123456,14-11-10,100,123456,20,bs
1,123456,10-11-12,100,123456,900,bs
1,123456,14-11-17,100,123456,700,bs
2,Next i wanted to get the sum(fare) of those rollno that were common between the 2 tables and also whose fare >= fees from FIRST table group by rollno and id.
My query is:
SELECT x.ID,x.rl_no,,x.adm_date,x.fees,x.rollno,x.type,sum(x.fare) as "fare" from (SELECT a.ID,a.rl_no,a.adm_date,a.fees,b.rollno,b.fare,b.type FROM FIRST a
INNER JOIN
SECOND b ON a.rl_no = b.rollno) x, FIRST y
WHERE x.rollno = y.rl_no AND x.fare >= y.fees AND x.type IS NOT NULL GROUP BY x.rollno,x.ID ;
But this is throwing in exceptions.
ORA-00979: not a GROUP BY expression
00979. 00000 - "not a GROUP BY expression"
The expected output will be like this:
id,rollno,adm_date,fare,type
1,123456,14-11-10,1620,bs
So could someone care to show an oracle newbie what i'm doing wrong here?

It looks like there's a couple different problems here;
Firstly, you're trying to group by an x.ID column which doesn't exist; it looks like you'll want to add ID to the selected columns in your sub-query.
Secondly, when aggregating with GROUP BY, all selected columns need to be either listed in the GROUP BY statement or aggregated. If you're grouping by rollno and ID, what do you want to have happen to all the extra values for adm_date, fees, and type? Are those always going to be the same for each distinct rollno and ID pair?
If so, simply add them to the GROUP BY statement, ie,
GROUP BY adm_date, fees, type, rollno, ID
If not, you'll need to work out exactly how you want to select which one to be output; If you've got output like your example (adding in an ID column here)
ID,adm_date,fees,rollno,fare,type
1,14-11-10,100,123456,20,bs
1,10-11-12,100,123456,900,bs
1,14-11-17,100,123456,700,bs
Call that result set 'a'. If I run;
SELECT a.ID, a.rollno, SUM(a.fare) as total_fare
FROM a
GROUP BY a.ID, a.rollno
Then the result will be a single row;
ID,rollno,total_fare
1,123456,1620
So, if you also select the adm_date, fees, and type columns, oracle has no idea what you mean to do with them. You're not using them for grouping, and you're not telling oracle how you want to pick which one to use.
You could do something like
SELECT a.ID,
FIRST(a.adm_date) as first_adm_date,
FIRST(a.fees) as first_fees,
a.rollno,
SUM(a.fare) as total_fare,
FIRST(a.type) as first_type
FROM a
GROUP BY a.ID, a.rollno
Which would give the result;
ID,first_adm_date,first_fees,rollno,total_fare,first_type
1,14-11-10,100,123456,1620,bs
I'm not sure if that's what you mean to do though.

What does a column assignment using an aggregate in the columns area of a select do?

I'm trying to decipher another programmer's code who is long-gone, and I came across a select statement in a stored procedure that looks like this (simplified) example:
SELECT #Table2.Col1, Table2.Col2, Table2.Col3, MysteryColumn = CASE WHEN y.Col3 IS NOT NULL THEN #Table2.MysteryColumn - y.Col3 ELSE #Table2.MysteryColumn END
INTO #Table1
FROM #Table2
LEFT OUTER JOIN (
SELECT Table3.Col1, Table3.Col2, Col3 = SUM(#Table3.Col3)
FROM Table3
INNER JOIN #Table4 ON Table4.Col1 = Table3.Col1 AND Table4.Col2 = Table3.Col2
GROUP BY Table3.Col1, Table3.Col2
) AS y ON #Table2.Col1 = y.Col1 AND #Table2.Col2 = y.Col2
WHERE #Table2.Col2 < #EnteredValue
My question, what does the fourth column of the primary selection do? does it produce a boolean value checking to see if the values are equal? or does it set the #Table2.MysteryColumn equal to some value and then inserts it into #Table1? Or does it just update the #Table2.MysteryColumn and not output a value into #Table1?
This same thing seems to happen inside of the sub-query on the third column, and I am equally at a loss as to what that does as well.

MysteryColumn = gives the expression a name also called a column alias. The fact that a column in the table#2 also has the same name is besides the point.
Since it uses INTO syntax it also gives the column its name in the resulting temporary table. See the SELECT CLAUSE and note | column_alias = expression and the INTO CLAUSE

comprare aggregate sum function to number in postgres

I have the next query which does not work:
UPDATE item
SET popularity= (CASE
WHEN (select SUM(io.quantity) from item i NATURAL JOIN itemorder io GROUP BY io.item_id) > 3 THEN TRUE
ELSE FALSE
END);
Here I want to compare each line of inner SELECT SUM value with 3 and update popularity. But SQL gives error:
ERROR: more than one row returned by a subquery used as an expression
I understand that inner SELECT returns many values, but can smb help me in how to compare each line. In other words make loop.

When using a subquery you need to get a single row back, so you're effectively doing a query for each record in the item table.
UPDATE item i
SET popularity = (SELECT SUM(io.quantity) FROM itemorder io
WHERE io.item_id = i.item_id) > 3;
An alternative (which is a postgresql extension) is to use a derived table in a FROM clause.
UPDATE item i2
SET popularity = x.orders > 3
FROM (select i.item_id, SUM(io.quantity) as orders
from item i NATURAL JOIN itemorder io GROUP BY io.item_id)
as x(item_id,orders)
WHERE i2.item_id = x.item_id
Here you're doing a single group clause as you had, and we're joining the table to be updated with the results of the group.

Prepare dynamic case statement using PostgreSQL 9.3

I have the following case statement to prepare as a dynamic as shown below:
Example:
I have the case statement:
case cola
when cola between '2001-01-01' and '2001-01-05' then 'G1'
when cola between '2001-01-10' and '2001-01-15' then 'G2'
when cola between '2001-01-20' and '2001-01-25' then 'G3'
when cola between '2001-02-01' and '2001-02-05' then 'G4'
when cola between '2001-02-10' and '2001-02-15' then 'G5'
else ''
end
Note: Now I want to create dynamic case statement because of the values dates and name passing as a parameter and it may change.
Declare
dates varchar = '2001-01-01to2001-01-05,2001-01-10to2001-01-15,
2001-01-20to2001-01-25,2001-02-01to2001-02-05,
2001-02-10to2001-02-15';
names varchar = 'G1,G2,G3,G4,G5';
The values in the variables may change as per the requirements, it will be dynamic. So the case statement should be dynamic without using loop.

You may not need any function for this, just join to a mapping data-set:
with cola_map(low, high, value) as (
values(date '2001-01-01', date '2001-01-05', 'G1'),
('2001-01-10', '2001-01-15', 'G2'),
('2001-01-20', '2001-01-25', 'G3'),
('2001-02-01', '2001-02-05', 'G4'),
('2001-02-10', '2001-02-15', 'G5')
-- you can include as many rows, as you want
)
select table_name.*,
coalesce(cola_map.value, '') -- else branch from case expression
from table_name
left join cola_map on table_name.cola between cola_map.low and cola_map.high
If your date ranges could collide, you can use DISTINCT ON or GROUP BY to avoid row duplication.
Note: you can use a simple sub-select too, I used a CTE, because it's more readable.
Edit: passing these data (as a single parameter) can be achieved by passing a multi-dimensional array (or an array of row-values, but that requires you to have a distinct, predefined composite type).
Passing arrays as parameters can depend on the actual client (& driver) you use, but in general, you can use the array's input representation:
-- sql
with cola_map(low, high, value) as (
select d[1]::date, d[2]::date, d[3]
from unnest(?::text[][]) d
)
select table_name.*,
coalesce(cola_map.value, '') -- else branch from case expression
from table_name
left join cola_map on table_name.cola between cola_map.low and cola_map.high
// client pseudo code
query = db.prepare(sql);
query.bind(1, "{{2001-01-10,2001-01-15,G2},{2001-01-20,2001-01-25,G3}}");
query.execute();
Passing each chunk of data separately is also possible with some clients (or with some abstractions), but this is highly depends on your driver/orm/etc. you use.