Order by -- complex sorting?

Question

Hello:
How can one accomplish the following:
Let's say I have the following scenario:
Select value2 from table2 IF this value exists. Else, select value 1 from table1. --
Then: I would like to ORDER BY all of the above results (sometimes the value may be value1, other times vaule2, etc). How can one do such an ORDER BY? Is this even possible via a query?

Answer 1

Hi,
In most cases, you can use functions and CASE expressions (or more specialized if-then-else expressions, like COALESCE) to sort things the way you want.
For example, if you want to pass a parameter, :p_sort_key, a VARCHAR2 that tells if you want to sort by value1, value2or value3, then you can do this:
ORDER BY  CASE  :p_sort_key
          WHEN  'V1'  THEN value1
          WHEN  'V2'  THEN value2
          WHEN  'V3'  THEN value3
             ENDThis assumes value1, value2 and value3 all hafve the same datatype. If not, use TO_CHAR or some other conversion function on one or more of them.
Now say we want to sort by table2.value4 if it is present, and otherwise by the expression above.
Depending on your exact requirements, you could say
ORDER BY  table2.value4               NULLS LAST
,            CASE  :p_sort_key
          WHEN  'V1'  THEN value1
          WHEN  'V2'  THEN value2
          WHEN  'V3'  THEN value3
             ENDThis will sort by table2.value4, and then, as a tie-breaker, by the other value. (NULLS LAST is the default, so you don't have to say explicitly say it, but you can for clarity's sake.)
if you really want the second expression only to have effect when table2.value2 is NULL, then you could use a NVL statement, or incorporate value4 into the CASE:
ORDER BY  CASE 
          WHEN  table2.value4  IS     NOT NULL
                                      THEN table2.value4
          WHEN  :p_sort_key = 'V1'  THEN value1
          WHEN  :p_sort_key = 'V2'  THEN value2
          WHEN  :p_sort_key = 'V3'  THEN value3
             ENDIf all else fails, there's always dynamic SQL.
In SQL*Plus, for example, you could run a preliminary query that produces a sting like
SUBSTR (value1, 1, 2),
value3,
TRUNC (entry_date, 'YYYY') and sets the substitution variable &order_txt to that value.
Then, in your main query, you say:
ORDER BY  &order_txt