Oracle 12c - SQL 字符串构建问题

Question

这是对上一个问题的跟进：Dynamically build select statement in Oracle 12c

我正在尝试动态构建 select 语句，但在构建列别名时遇到问题。必须从 MAIN_TABLE 中检索列别名。请参考下面里面的代码：

declare
    upper_level number;
    t_sql varchar2(1000);
    t_sql_val varchar2(500);
    l_sql varchar2(1000);
begin

--upper_level will always be given
    select 3 into upper_level from dual;

--build the fixed string
    l_sql:='SELECT ID,
     Title,
     Desc,
     Type,';


    for lvl in 1..upper_level
     loop
--build the column names and alias names
         t_sql:=t_sql||'TYPE_'||lvl||' <SELECT TYPE_'||lvl||' FROM MAIN_TABLE WHERE ID = 1>,';

     end loop;

--finish building the statement     
     t_sql:=rtrim(t_sql,',');
     l_sql:=l_sql||t_sql;
     l_sql:=l_sql||' FROM SCHEMA.TABLE
    WHERE ID = 1;';

--dbms_output.put_line(l_sql);

end;

这是我的尝试：

declare
    upper_level number;
    t_sql varchar2(1000);
    v_sql varchar2(1000);
    v_sql_val varchar2(1000);
    t_sql_val varchar2(500);
    l_sql varchar2(1000);
begin

--upper_level will always be given
    select 3 into upper_level from dual;

--build the fixed string
    l_sql:='SELECT ID,
     Title,
     Desc,
     Type,';


    for lvl in 1..upper_level
     loop
--build the column names and alias names
         t_sql:='TYPE_'||lvl||;
         v_sql:='SELECT '||t_sql||' FROM MAIN_TABLE WHERE ID=1';
         EXECUTE IMMEDIATE v_sql into v_sql_val;
         t_sql:=t_sql||' '||v_sql_val||',';
     end loop;

--finish building the statement     
     t_sql:=rtrim(t_sql,',');
     l_sql:=l_sql||t_sql;
     l_sql:=l_sql||' FROM SCHEMA.TABLE
    WHERE ID = 1;';

dbms_output.put_line(l_sql);

end;

我的尝试结果：

SELECT ID,
     Title,
     Desc,
     Type,TYPE_3 LVL_3 FROM SCHEMA.TABLE
    WHERE ID = 1;

我的预期结果：

SELECT ID,
     Title,
     Desc,
     Type,TYPE_1 LVL_1, TYPE_2 LVL_2, TYPE_3 LVL_3 FROM SCHEMA.TABLE
    WHERE ID = 1;

我怎样才能做到这一点？

Answer 1

我已经想出了如何达到预期的效果：

declare
    upper_level number;
    t_sql varchar2(1000);
    v_sql varchar2(1000);
    v_sql_val varchar2(1000);
    t_sql_val varchar2(500);
    l_sql varchar2(1000);
begin

--upper_level will always be given
    select 3 into upper_level from dual;

--build the fixed string
    l_sql:='SELECT ID,
     Title,
     Desc,
     Type,';


    for lvl in 1..upper_level
     loop
--build the column names and alias names
     t_sql:=t_sql||'TYPE_'||lvl||'_CD';
     v_sql:='SELECT TYPE_'||lvl||'_CD FROM MAIN_TABLE WHERE ID=1';
     EXECUTE IMMEDIATE v_sql into v_sql_val;
     t_sql:=t_sql||' '||v_sql_val||',';
     end loop;

--finish building the statement     
     t_sql:=rtrim(t_sql,',');
     l_sql:=l_sql||t_sql;
     l_sql:=l_sql||' FROM SCHEMA.TABLE
    WHERE ID = 1;';

dbms_output.put_line(l_sql);

end;