EXECUTE IMMEDIATE '(select * from table where code =abc)union all(select * from table where code =bcd)union all(select * from table where code =cde)'

------解决方案--------------------
这条是找括号位置的：

SQL code
SELECT INSTR('(SELECT * FROM A WHERE CODE IN (SELECT CODE FROM B))', ')') AS POINTLEFT FROM DUAL)

------解决方案--------------------
用存储过程吧：

传入存储过程一个参数：你的SQL语句字符串！
存储过程输出一个结果：游标

------解决方案--------------------
没太看懂你要的结果，给出个过程的结构，自己把逻辑填写在
  /*sql处理代码*/这个地方吧。
SQL code

create or replace procedure test(result_message OUT VARCHAR2) is
result_sum varchar2(10);
result_avg varchar2(10);
result_sqrt varchar2(10);

begin
  /*sql处理代码*/
  
  result_message:=result_sum+result_avg+result_sqrt;
end test;

------解决方案--------------------
整个游标
SQL code
select substrb(rn,instr(rn,'(',1)+1)from
(select regexp_substr('sum(abc)+avg(bcd)+sqrt(cde)','[^)]+',1,rownum) rn from dual connect by rownum<=length(regexp_replace('sum(abc)+avg(bcd)+sqrt(cde)','[^)]+')) 
)

------解决方案--------------------
这个直接一条语句不就返回了吗？加 UNION ALL 啊？

SQL code
(select * from table where code =abc)
UNION ALL
(select * from table where code =bcd)
UNION ALL
(select * from table where code =cde)

------解决方案--------------------
给来个最简单的  
SQL code

select instr('1111112','1',-1)  from dual