用hierachical query，即connect by配合dual表生成序列，mod这个是取余函数，生成group factor。最后面的connect by rownum <= 100可以替换为connect by level <= 100
select case when mod(rownum,200000) = 0 then 5
else mod(rownum,4)
end X,
LEVEL Lvl,
rpad( 'x', 100, 'x' ) data
from dual
connect by rownum <= 100