Question

I’m emulating the DESCRIBE tablename available in Sqlplus to make similiar functionality available via regular sql.

I have the objects/function listed below, and it works – except for the order_param clause. It’s basically ignored. I can call the function via:

select * from table(describe('my_table',1));

or

select * from table(describe('my_table',2));

and it results in the same ordering.

If I replace order by order_param with either order by 1 or order by 2 in the sql clause in the function, it works as expected.

There must be a better way to get this to work other than to use dynamic sql (or have the function actually choose between 2 sql clauses).

CREATE OR REPLACE TYPE table_description as object (
  column_name varchar2(30),
  column_id   number, 
  data_type   varchar2(30),
  nullable    varchar2(1)
);

CREATE OR REPLACE TYPE table_description_type as table of table_description;

CREATE OR REPLACE FUNCTION describe (tname IN VARCHAR2, order_param in NUMBER default 2) RETURN table_description_type AS
-- function to describe a table
-- eg. use via select * from table(describe('table_name'));

  v_ret table_description_type;

BEGIN

 select cast(multiset(
    select column_name, data_type || data_suff "Data Type", nullable from (
      select column_name, column_id, data_type, 
      case when data_type = 'DATE' then '' else '(' ||  
        case when data_type in ('CHAR','VARCHAR2','NCHAR','NVARCHAR') 
        then to_char(char_length) else data_precision || ', ' || data_scale end || ')'
      end data_suff, 
      nullable, data_default, num_nulls, last_analyzed, avg_col_len 
      from all_tab_columns where table_name = upper(tname)
      order by order_param
    )
 ) as table_description_type) into v_ret from dual;

 return v_ret;

END;
/

Answer 1

You can’t do this the way you’re attempting to. In order for the order clause to interpret a numeric value as a column reference, the number has to be there when the query is parsed. Since you’re binding the variable in, it will always interpret your parameter as a value, not a column reference. To fix this, you’ll want to change the order by clause logically to select a column based on the value:

order by case order_param 
         when 1 then column_name 
         else data_type || data_suff
         end