Joining two rows with same date/time - sql

Question

I've got a working query which is fine for now - and just about does what I'm looking for. I'm wanting to consult, however, as to whether this is the most sensible way of manipulating my data to have it spit out what I need:
I've got a table REPORTS which stores report data. One row gets inserted when a report is run, and another when a report is confirmed. Confirming a report simply involves inserting a reserved name TRUE with the same date as the report to be confirmed. Ugly, yes. But unfortunately, it's not up to me to decide...
Table structure:
Reports
UID (char)
Report (char)
Date (date)
On having run a report, the table REPORTS might look a little like this:
+------+--------+---------------------+
| UID | Report | Date |
+------+--------+---------------------+
| 0001 | runX | 2014-01-02 03:04:59 |
| 0001 | runY | 2014-01-02 03:05:58 |
| 0001 | runX | 2014-01-02 03:06:20 |
+------+--------+---------------------+
On action 'report confirm', the following rows would be inserted:
+------+--------+---------------------+
| UID | Report | Date |
+------+--------+---------------------+
| 0001 | TRUE | 2014-01-02 03:04:59 |
| 0001 | TRUE | 2014-01-02 03:05:58 |
| 0001 | TRUE | 2014-01-02 03:06:20 |
+------+--------+---------------------+
As you can see, when a report is marked TRUE (ie correct), there are two rows with exactly the same DATE:
+------+--------+---------------------+
| UID | Report | Date |
+------+--------+---------------------+
| 0001 | runX | 2014-01-02 03:04:59 |
| 0001 | TRUE | 2014-01-02 03:04:59 |
| 0001 | runY | 2014-01-02 03:05:58 |
| 0001 | TRUE | 2014-01-02 03:05:58 |
| 0001 | runX | 2014-01-02 03:06:20 |
| 0001 | TRUE | 2014-01-02 03:06:20 |
+------+--------+---------------------+
To return all reports which are 'correct' ie TRUE and identical date/time to report name eg 'runX', I do the following:
SELECT * FROM REPORTS T1
LEFT JOIN REPORTS T2
ON T1.DATE = T2.DATE
WHERE T1.REPORT = 'TRUE'
AND T1.REPORT != T2.REPORT;
This gives me something I can at least work with. I know, however, that there must be a more elegant way of doing this? The last clause, for example: not putting that in has it spit out a cartesian product, meaning I've created a cartesian product and am then filtering it. Presumably there must be a way of avoiding it completely and not creating it in the first place?

Answer 1

If I understand correctly, you want to pull the name from the record at the same time as the TRUE record and only return reports that actually have a TRUE record:
select uid,
max(case when Report <> 'TRUE' then Report end) as Report,
date
from reports r
group by uid, date
having sum(case when Report = 'TRUE' then 1 else 0 end) > 0;
Note: Doing equality comparisons on dates with a time component seems dangerous. The process that creates these tables should be putting some other link to the right report in the record. For instance, it could update a check flag column rather than create a new row.
EDIT:
Why is joining on dates (with times) as bad idea? Often, dates are shown as only dates, without the time component. That means that two dates can look the same in output, but really be different. Or, two dates can be in different time zones and look different but be the same.
Oracle mitigates the first problem by storing dates up to the second, in an exact format. Two dates that look the same to the second are the same. Equivalent data types in other databases sometimes include milliseconds -- although these are rarely printed out with the value. Two dates with times up to the second can look the same and still be different. In Oracle, you could say that two dates with times up to the minute can look the same and still be different.
The same phenomenon happens with floating point data types -- 1.0000000 and 0.9999999 are different, but they look the same when shown as 1.000. A join on these values would fail, even though looking at the values would suggest that it would succeed.

Answer 2

SQL Fiddle
Oracle 11g R2 Schema Setup:
CREATE TABLE reports ( "UID", Report, "Date" ) AS
SELECT '0001', 'runX', TO_DATE( '2014-01-02 03:04:59', 'yyyy-mm-dd hh24:mi:ss' ) FROM DUAL
UNION ALL SELECT '0001', 'TRUE', TO_DATE( '2014-01-02 03:04:59', 'yyyy-mm-dd hh24:mi:ss' ) FROM DUAL
UNION ALL SELECT '0001', 'runY', TO_DATE( '2014-01-02 03:05:58', 'yyyy-mm-dd hh24:mi:ss' ) FROM DUAL
UNION ALL SELECT '0001', 'TRUE', TO_DATE( '2014-01-02 03:05:58', 'yyyy-mm-dd hh24:mi:ss' ) FROM DUAL
UNION ALL SELECT '0001', 'runX', TO_DATE( '2014-01-02 03:06:20', 'yyyy-mm-dd hh24:mi:ss' ) FROM DUAL
UNION ALL SELECT '0001', 'TRUE', TO_DATE( '2014-01-02 03:06:20', 'yyyy-mm-dd hh24:mi:ss' ) FROM DUAL;
Query 1:
SELECT "UID",
MAX( CASE Report WHEN 'TRUE' THEN NULL ELSE Report END ) AS Report,
"Date"
FROM reports
GROUP BY "UID", "Date"
HAVING MAX( CASE Report WHEN 'TRUE' THEN 1 ELSE 0 END ) = 1
Results:
| UID | REPORT | DATE |
|------|--------|--------------------------------|
| 0001 | runX | January, 02 2014 03:04:59+0000 |
| 0001 | runY | January, 02 2014 03:05:58+0000 |
| 0001 | runX | January, 02 2014 03:06:20+0000 |
Query 2:
Assuming that when a report is marked as FALSE if it is not correct then you could do:
SELECT "UID",
Report,
"Date"
FROM ( SELECT "UID",
Report,
LEAD( Report )
OVER (
PARTITION BY "UID", "Date"
ORDER BY CASE Report
WHEN 'TRUE' THEN 2
WHEN 'FALSE' THEN 1
ELSE 0
END ) AS Result,
"Date"
FROM Reports )
WHERE Result = 'TRUE'
Results:
| UID | REPORT | DATE |
|------|--------|--------------------------------|
| 0001 | runX | January, 02 2014 03:04:59+0000 |
| 0001 | runY | January, 02 2014 03:05:58+0000 |
| 0001 | runX | January, 02 2014 03:06:20+0000 |