Left outer join using date range returns too many rows

Question

I am trying to pull data for a website.
Names table:
company_name varchar2(30)
julian_day varchar2(3)
logins number(3)
login_errors number(3)
Given a julian date range (e.g. 250-252), I am displaying the information from the Names table.
The problem is that I also need to display changes (increases/decreases) in the data. I do that by coloring the text based on a 10% increase/decrease. Data for the 3 days 250-252 would be compared to data for the previous 3 days 247-249.
Not all companies will report data on all days, so some gaps in the data may exist. Therefore, I cannot do just a simple join.
I am trying to write a query that will give me this information if the user chooses days 250-252.
Company_name
sum(logins) for days 250-252
sum(login_errors) for days 250-252
sum(logins) for days 247-249
sum(login_errors) for days 247-249
The query I'm using is:
select cur.company_name, sum(cur.logins),
sum(cur.login_errors), sum(old.logins), sum(old.login_errors)
FROM names cur LEFT OUTER JOIN names old
ON cur.company_name = old.company_name
WHERE cur.adate>='250' and cur.adate<='252'
and old.adate>='247' and old.adate<='249'
GROUP by cur.company_name
Given this data:
Company_name adate logins login_errors
ABC 247 10 10
ABC 248 20 20
ABC 249 30 30
ABC 250 15 15
ABC 251 25 25
ABC 252 35 35
My problem is that it returns:
adate cur.logins cur.login_err old.logins old.login_err
250 15 15 60 60
251 25 25 60 60
252 35 35 60 60
How can I get it to only give me the one "old" day's data? I went with the LEFT OUTER JOIN because it's possible that there is no data for an "old" day.
Thanks in advance.....

Answer 1

Your problem stems from the join itself, and would be the same even without the OUTER JOIN. With your data, there are 3 records in cur and 3 records in old. The join matches each record in cur to each record in old resulting in 9 records in total. Without the SUM, this is clear:
SQL> SELECT cur.company_name, cur.logins, cur.login_errors,
  2         old.logins, old.login_errors, cur.adate cad, old.adate oad
  3  FROM names cur LEFT OUTER JOIN names old
  4                 ON cur.company_name = old.company_name
  5  WHERE cur.adate>=250 and cur.adate<=252 and
  6        old.adate>=247 and old.adate<=249;
COMPANY_NA     LOGINS LOGIN_ERRORS     LOGINS LOGIN_ERRORS        CAD        OAD
ABC                35           35         10           10        252        247
ABC                25           25         10           10        251        247
ABC                15           15         10           10        250        247
ABC                35           35         20           20        252        248
ABC                25           25         20           20        251        248
ABC                15           15         20           20        250        248
ABC                35           35         30           30        252        249
ABC                25           25         30           30        251        249
ABC                15           15         30           30        250        249
9 rows selected.You can do this with only one reference to the table.
SELECT company_name,
       SUM(CASE WHEN adate BETWEEN 250 and 252 THEN logins ELSE 0 END) curlog,
       SUM(CASE WHEN adate BETWEEN 250 and 252 THEN login_errors ELSE 0 END) curerr,
       SUM(CASE WHEN adate BETWEEN 247 and 249 THEN logins ELSE 0 END) oldlog,
       SUM(CASE WHEN adate BETWEEN 247 and 249 THEN login_errors ELSE 0 END) olderr
FROM names
WHERE adate BETWEEN 247 and 252
GROUP BY company_nameHTH
John