Decode in order by - asc/desc

Question

I'm trying to be able to dynamically change the asc/desc in the order by.
Here is some sql that works:
select * from transactions order by decode(somevar,'a',acct_nbr,'c',card_number,acct_nbr) asc
What I wanted was something like this:
select * from transactions order by decode(somevar,'a',acct_nbr,'c',card_number,acct_nbr) decode(anothervar,'a',asc,'d',desc,asc)
but this doesn't appear to work. Am I missing something, or is there another way.
Thanks in advance,

Answer 1

There are a bunch of restrictions on ordering when you use union all. It usually either requires the numerical position of the column, so if you want to order by the first column, then you order by 1, or a column alias, which sometimes can be used in only the first select and is sometimes required in all of them. However, even if you order by column position, such as order by 1, it does not seem to allow this in a decode. The simplest method is to use your entire query containing the union all as an inline view in an outer query. Please see the following examples that do this and also demonstrate how to get the order by options, including ascending and descending that you desire, assuming that your columns are of numeric data types.
scott@ORA92> -- sort by acct_nbr asc:
scott@ORA92> ACCEPT somevar PROMPT 'Enter a to sort by acct_nbr or c to sort by card_number: '
Enter a to sort by acct_nbr or c to sort by card_number: a
scott@ORA92> ACCEPT anothervar PROMPT 'Enter 1 to sort ascending or -1 to sort descending: '
Enter 1 to sort ascending or -1 to sort descending: 1
scott@ORA92> SELECT acct_nbr, card_number
  2  FROM   (SELECT 1     AS acct_nbr,
  3                3     AS card_number
  4            FROM   dual
  5            UNION ALL
  6            SELECT 2     AS acct_nbr,
  7                4     AS card_number
  8            FROM   dual
  9            UNION ALL
10            SELECT 3     AS acct_nbr,
11                2     AS card_number
12            FROM   dual
13            UNION ALL
14            SELECT 4     AS acct_nbr,
15                5     AS card_number
16            FROM   dual
17            UNION ALL
18            SELECT 5     AS acct_nbr,
19                1     AS card_number
20            FROM   dual)
21  ORDER  BY DECODE ('&somevar',
22                   'a', acct_nbr * NVL (&anothervar, 1),
23                   'c', card_number * NVL (&anothervar, 1),
24                   acct_nbr * NVL (&anothervar, 1))
25  /
  ACCT_NBR CARD_NUMBER
         1           3
         2           4
         3           2
         4           5
         5           1
scott@ORA92> -- sort by card_number asc:
scott@ORA92> ACCEPT somevar PROMPT 'Enter a to sort by acct_nbr or c to sort by card_number: '
Enter a to sort by acct_nbr or c to sort by card_number: c
scott@ORA92> ACCEPT anothervar PROMPT 'Enter 1 to sort ascending or -1 to sort descending: '
Enter 1 to sort ascending or -1 to sort descending: 1
scott@ORA92> SELECT acct_nbr, card_number
  2  FROM   (SELECT 1     AS acct_nbr,
  3                3     AS card_number
  4            FROM   dual
  5            UNION ALL
  6            SELECT 2     AS acct_nbr,
  7                4     AS card_number
  8            FROM   dual
  9            UNION ALL
10            SELECT 3     AS acct_nbr,
11                2     AS card_number
12            FROM   dual
13            UNION ALL
14            SELECT 4     AS acct_nbr,
15                5     AS card_number
16            FROM   dual
17            UNION ALL
18            SELECT 5     AS acct_nbr,
19                1     AS card_number
20            FROM   dual)
21  ORDER  BY DECODE ('&somevar',
22                   'a', acct_nbr * NVL (&anothervar, 1),
23                   'c', card_number * NVL (&anothervar, 1),
24                   acct_nbr * NVL (&anothervar, 1))
25  /
  ACCT_NBR CARD_NUMBER
         5           1
         3           2
         1           3
         2           4
         4           5
scott@ORA92> -- sort by acct_nbr desc:
scott@ORA92> ACCEPT somevar PROMPT 'Enter a to sort by acct_nbr or c to sort by card_number: '
Enter a to sort by acct_nbr or c to sort by card_number: a
scott@ORA92> ACCEPT anothervar PROMPT 'Enter 1 to sort ascending or -1 to sort descending: '
Enter 1 to sort ascending or -1 to sort descending: -1
scott@ORA92> SELECT acct_nbr, card_number
  2  FROM   (SELECT 1     AS acct_nbr,
  3                3     AS card_number
  4            FROM   dual
  5            UNION ALL
  6            SELECT 2     AS acct_nbr,
  7                4     AS card_number
  8            FROM   dual
  9            UNION ALL
10            SELECT 3     AS acct_nbr,
11                2     AS card_number
12            FROM   dual
13            UNION ALL
14            SELECT 4     AS acct_nbr,
15                5     AS card_number
16            FROM   dual
17            UNION ALL
18            SELECT 5     AS acct_nbr,
19                1     AS card_number
20            FROM   dual)
21  ORDER  BY DECODE ('&somevar',
22                   'a', acct_nbr * NVL (&anothervar, 1),
23                   'c', card_number * NVL (&anothervar, 1),
24                   acct_nbr * NVL (&anothervar, 1))
25  /
  ACCT_NBR CARD_NUMBER
         5           1
         4           5
         3           2
         2           4
         1           3
scott@ORA92> -- sort by card_number desc:
scott@ORA92> ACCEPT somevar PROMPT 'Enter a to sort by acct_nbr or c to sort by card_number: '
Enter a to sort by acct_nbr or c to sort by card_number: c
scott@ORA92> ACCEPT anothervar PROMPT 'Enter 1 to sort ascending or -1 to sort descending: '
Enter 1 to sort ascending or -1 to sort descending: -1
scott@ORA92> SELECT acct_nbr, card_number
  2  FROM   (SELECT 1     AS acct_nbr,
  3                3     AS card_number
  4            FROM   dual
  5            UNION ALL
  6            SELECT 2     AS acct_nbr,
  7                4     AS card_number
  8            FROM   dual
  9            UNION ALL
10            SELECT 3     AS acct_nbr,
11                2     AS card_number
12            FROM   dual
13            UNION ALL
14            SELECT 4     AS acct_nbr,
15                5     AS card_number
16            FROM   dual
17            UNION ALL
18            SELECT 5     AS acct_nbr,
19                1     AS card_number
20            FROM   dual)
21  ORDER  BY DECODE ('&somevar',
22                   'a', acct_nbr * NVL (&anothervar, 1),
23                   'c', card_number * NVL (&anothervar, 1),
24                   acct_nbr * NVL (&anothervar, 1))
25  /
  ACCT_NBR CARD_NUMBER
         4           5
         2           4
         1           3
         3           2
         5           1
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