How to use column value as select expression

问题

There is a table that contains a column containing a SQL select expression. See the following example:

╔════╦════════╦═══════════════════════════════════════════╗
║ ID ║ Code   ║ Expression                                ║
║ 1  ║ B12321 ║ SELECT * FROM table WHERE code LIKE '%'   ║
║ 2  ║ A35525 ║ SELECT * FROM table WHERE code = '1234'   ║
║ 3  ║ C23213 ║ SELECT * FROM table WHERE code <> '%D'    ║
╚════╩════════╩═══════════════════════════════════════════╝

I want to loop throught the Expression column, execute those statements and insert the result into another table. Is this even possible? I can't find related questions about that. Besides that, I read about a using a cursor to loop throught the table but there is a lot of negativity about that.

If possible, can you provide usefull links about the question I have, or even better a sample code doing this.

回答1:

Yes, possible by using dynamic SQL in which qualify excerpt code with colon as :code, and use double loop as below :

SQL> desc tab2
Name       Type           
---------- -------------  
ID         VARCHAR2(20)                          
NAME       VARCHAR2(100) 

SQL> create table tab3( ID varchar2(20), name varchar2(50));

SQL> declare
    v_row       tab2%rowtype;   
    v_cursor    sys_refcursor;
begin
   for c in ( select ID, code, expression from tab1 ) 
   loop  
    open v_cursor for replace(c.expression,' code ',' :code ') using c.code;
    loop
        fetch v_cursor
         into  v_row;
       exit when v_cursor%notfound;
     insert into tab3 values( v_row.id,v_row.name );   
    end loop;  
    close v_cursor;
   end loop; 
end;   
/

where tab2 is the common table within the expression column.

回答2:

You want dynamic SQL. As you don't want to return any data, this is possible with a small PL/SQL procedure:

create or replace procedure insert_by_code(in_code varchar2) as
begin
  for rec in (select expression from mytable where code = in_code) loop
    execute immediate 'insert into other_table ' || rec.expression;
    commit;
  end loop;
end insert_by_code;

or ad hoc with an anonymous block:

begin
  for rec in (select expression from mytable where code = :code) loop
    execute immediate 'insert into other_table ' || rec.expression;
    commit;
  end loop;
end;

来源：https://stackoverflow.com/questions/59078119/how-to-use-column-value-as-select-expression

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sql

Oracle

plsql

expression