How to JOIN the results of multiple SELECT statements

Question

Hello,
I currently have a table where each row represents a single census date for a given tag number, where each tag number can have multiple census dates and data which are represented by their own row in the table.
I currently have 2 SELECT statements that select all tag numbers given a census date, and would like to combine those 2 queries so that the data is presented in a manner such that for each tag number, instead of obtaining 2 rows for each tag number, I would be presented with 1 row for each tag number consisting of three columns: tag_number, data1, and data2.
Both of the requested dates are not necessarily present for all tag numbers, and in the event that only one date is present, the other column should contain null, but if both dates are not present for a given tag number, the tag number is omitted.
The two queries I would like to JOIN are:
SELECT a.tag_number, a.data AS data1
FROM tab1 a
WHERE
a.census_date='11/21/2009'and
SELECT b.tag_number, b.data AS data2
FROM tab2 b
WHERE
b.census_date='11/24/2009'The table from which I am selecting the data looks like such:
tag_number  census_date     data
    1       11/21/2009      10
    1       11/23/2009      11
    1       11/24/2009      12
    2       11/19/2009      13
    2       11/21/2009      14And the data that I would like to result looks like:
tag_number  data1           data2
    1       10              12
    2       14              nullAny help would be greatly appreciated.
Thank you in advance,
-Justin
Edited by: m8r-qbkka9 on Nov 24, 2009 4:17 PM

Answer 1

Hi,
This looks like a job for pivot
SELECT       a.tag_number
,       MAX (CASE  WHEN a.census_date = TO_DATE ('11/21/2009'
                                                   'MM/DD/YYYY'
               THEN a.data
            END
           )          AS data1
,       MAX (CASE  WHEN a.census_date = TO_DATE ('11/24/2009'
                                                   'MM/DD/YYYY'
               THEN a.data
            END
           )          AS data2
FROM      tab1    a
WHERE       a.census_date  IN ( TO_DATE ( '11/21/2009'
                                  , 'MM/DD/YYYY'
                   , TO_DATE ( '11/21/2009'
                                  , 'MM/DD/YYYY'
GROUP BY  a.tag_number
;Comparing DATEs to stirngs, like '11/24/2009', is simply asking for trouble.
Always use a conversion function (like TO_DATE) or DATE literals instead.