SQL equivalent of if-statement

Question

What would be really useful would be if I could do an SQL equivalent of an if-statement. I'm just making the following up to show what I mean:
SELECT CASE
          WHEN(det.user_entity_name = 'PO Header')
             THEN (SELECT pha.segment1
                     FROM po.po_headers_all pha
                    WHERE pha.po_header_id = ad.pk1_value)
          WHEN(det.user_entity_name = 'PO Line')
             THEN (SELECT pha.segment1
                     FROM po.po_headers_all pha
                        , po.po_lines_all pla
                    WHERE pha.po_header_id = pla.po_header_id
                      AND pla.po_line_id = ad.pk1_value)
          ELSE 'UNDEFINED'
       END doc_num
     , ad.seq_num
     , fu.description created_by
     , dt.description
     , det.user_entity_name
     , ad.creation_date
     , ad.entity_name
     , ad.pk1_value
     if dat.user_name = 'Short Text' then
     dst.short_text
     elseif dat.user_name = 'Long Text' then
     dlt.long_text
     end if
  FROM applsys.fnd_document_datatypes dat
     , applsys.fnd_document_entities_tl det
     , applsys.fnd_documents_tl dt
     , applsys.fnd_documents d
     , applsys.fnd_document_categories_tl dct
     , applsys.fnd_attached_documents ad
     if dat.user_name = 'Short Text' then
, applsys.fnd_documents_short_text dst
     elseif dat.user_name = 'Long Text' then
     , applsys.fnd_documents_long_text dlt
     end if            
     , applsys.fnd_user fu
WHERE d.document_id = ad.document_id
   AND dt.document_id = d.document_id
   AND dct.category_id = d.category_id
   AND d.datatype_id = dat.datatype_id
   AND ad.entity_name = det.data_object_code
     if dat.user_name = 'Short Text' then
AND dt.media_id = dst.media_id(+)
     elseif dat.user_name = 'Long Text' then
AND dt.media_id = dlt.media_id(+)
     end if  
   AND ad.created_by = fu.user_id
   AND d.creation_date > '01-OCT-2007'
     if dat.user_name = 'Short Text' then
AND dat.user_name = 'Short Text'
     elseif dat.user_name = 'Long Text' then
AND dat.user_name = 'Long Text'
     end if     
   AND dct.user_name = 'To Supplier';Is that possible, or am I just talking a load of nonsense?
Thanks

Answer 1

How to do IF in SQL:
1) CASE
2) DECODE
3) UNION, UNION ALL, MINUS, INTERSECT
4) AND + OR + ()
5) stored functions
6) ...
which is best just depends on the situation. For your example I would use decode in the select clause and some UNION ALL for the from/where clauses.