动态sql拼接单引号与 变量赋值

if (lower(s_table)='gl_detail') then 
     ms := 'select max(ts)  from ' || t_table;
    --给变量赋值
     execute immediate ms  into max_ts;
     --添加单引号
     max_ts := ''''||max_ts||'''';

     step_one_sql := 'delete from ' || t_table || ' where '|| pk_field || ' in (select ' || pk_field || ' from ' || s_table || ' where  '|| s_ts ||'>'||max_ts||')';   
     DBMS_OUTPUT.PUT_LINE(step_one_sql);     
     
   step_three_sql := 'insert into ' || t_table || ' select * from ' || s_table || ' where ' || s_ts;
   step_three_sql := step_three_sql || ' >'||max_ts;