PostgreSQL/plpython: how compare two columns from different table in loop - sql

Question

I have a problem with loop in which I must compare columns between different tables.
I have two tables year2004 and year2005. Both contains month numbers and an amount for that month. I want compare the amount from both tables and produce a third table year with the number of month and greatest amount for that month.
For example I have in 2004 - 100, in 2005 - 200 so I must return values(2005, number_of_month, 200). Have you any ideas for solve this problem?
PS. Sorry for my writing errors, I learned English only few years ago :)

Answer 1

I'm guessing that you're trying to find the greatest amount for each month across the two years.
This would be much, much easier if your data was all in one table monthly_statistics with a date column. Then it'd just be a simple aggregate function or a window.
So lets turn the two tables into one.
Given sample data:
CREATE TABLE year2004 ( month int primary key, amount int);
INSERT INTO year2004 (month, amount)
VALUES (1, 50), (2, 40), (3, 60), (4, 80), (5, 100), (6, 800), (7, 20), (8, 40), (9, 30), (10, 40), (11, 50), (12, 99);
CREATE TABLE year2005 ( month int primary key, amount int);
INSERT INTO year2005 (month, amount)
VALUES (1, 88), (2, 44), (3, 11), (4, 123), (5, 12), (6, 88), (7, 21), (8, 19), (9, 44), (10, 89), (11, 4), (12, 42);
we could either join the tables, or we could convert it to a single table by date then filter it. Here's how we might generate a single table with the contents:
SELECT DATE '2004-01-01' + month * INTERVAL '1' MONTH AS sampledate, amount
FROM year2004
UNION ALL
SELECT DATE '2005-01-01' + month * INTERVAL '1' MONTH, amount
FROM year2005;
That's what you'd use if you were going to create a new table, but if you don't care about the actual dates, only the months, you can simply union all the two tables:
WITH samples AS (
SELECT month, amount
FROM year2004
UNION ALL
SELECT month, amount
FROM year2005
)
SELECT month, max(amount) AS amount
FROM samples
GROUP BY 1
ORDER BY month;
samplemonth | amount
-------------+--------
5 | 123
11 | 89
1 | 99
2 | 88
3 | 44
9 | 40
4 | 60
6 | 100
10 | 44
12 | 50
7 | 800
8 | 21
(12 rows)