Question

I have the following setup:

TableA:
 id=1, type=Air
 id=2, type=Sea
 id=3, type=Land
 ... This is a dynamic list, so more could be added

TableB:
 id=42, tableA_Id=1, name=Falcon
 id=43, tableA_Id=1, name=Pigeon
 id=44, tableA_Id=2, name=Shark
 id=45, tableA_Id=3, name=Bear
 id=47, tableA_Id=3, name=Wolf
 ... This is a dynamic list, so more could be added

Desired Output:

SomeOtherTableHeader, Air, Sea, Land, SomeOtherTableHeader
----------------------------------------------------------
someOtherValue, Falcon, Shark, Bear, someOtherValue
someOtherValue, Falcon, Shark, Wolf, someOtherValue
someOtherValue, Pigeon, Shark, Bear, someOtherValue
someOtherValue, Pigeon, Shark, Wolf, someOtherValue

• Note that the TableA contents are now column headers
• Note that this query needs to generate a row for every combination of TableB values.

In other words, I am trying to read a list of TableA values and use them to represent column headers. I am trying to create rows based on the combination of values in TableB

Is something like this even possible to do in a query (without a stored procedure)? If so, what would be the best/easiest way to achieve something like this?

Thanks!

Answer 1

If the contents of tableA are dynamic, you cannot do this with a simple SQL query (since a query needs to have a static set of columns).

The usual approach is to write a stored procedure that generates your SQL statement on-the-fly; for simplicity, I’ve just used a PL/SQL block:

  -- setup tables + contents

create table tableA(id number, type varchar2(100));
create table tableB(id number, tableA_id number, name varchar2(100));
insert into tableA values(1, 'Air');
insert into tableA values(2, 'Sea');
insert into tableA values(3, 'Land');
insert into tableB values(42, 1, 'Falcon');
insert into tableB values(43, 1, 'Pigeon');
insert into tableB values(44, 2, 'Shark');
insert into tableB values(45, 3, 'Bear');
insert into tableB values(47, 3, 'Wolf');
commit;

-- build the query (we just print it, usually you'd use something like "open cursor for l_sql"
declare
  l_sql varchar2(4000);
  l_select  varchar2(4000);
  l_with varchar2(4000);
  l_subviewname varchar2(30);
  l_from varchar2(4000);
begin
  l_sql := ' with ';
  l_select := ' select ';
  l_from := ' from ';

  for cur in (select * from tableA order by id)
  loop 
    l_subviewname := 'v_' || cur.type;
    l_with := l_with || l_subviewname || ' as (select b.* from tableB b, tableA a where b.tableA_id = a.id and a.type = ''' || cur.type || '''),' ||  chr(10);
    l_select := l_select || cur.type || '.name as ' || cur.type || ',';
    l_from := l_from || l_subviewname || ' ' || cur.type || ',';
  end loop;
  -- get rid of trailing ,
  l_with := regexp_replace(l_with, ',\s*$', '');
  l_select := regexp_replace(l_select, ',\s*$', '');
  l_from := regexp_replace(l_from, ',\s*$', '');

  l_sql := l_sql || l_with || l_select || l_from;
  dbms_output.put_line(l_sql);
end;