为整数中的每个数字生成行

Question

我有一张 ROADS 表：

with cte as (
select      1 as road_id,      2 as num_vertices    from dual
union all
select      2 as road_id,   null as num_vertices    from dual
union all
select      3 as road_id,   1000 as num_vertices    from dual
)
select
    *
from
    cte

   ROAD_ID NUM_VERTICES
---------- ------------
         1            2
         2         null
         3         1000

---

对于每条道路，我想为每个顶点生成行：

   ROAD_ID       VERTEX
---------- ------------
         1            1
         1            2
         3            1
         3            2
         3          ...
         3         1000  

---

问题：

有没有办法在 SELECT 查询中为每个 NUM_VERTICES 整数生成行？

Answer 1

CONNECT BY 和 CROSS JOIN 似乎也可以解决问题：

select level as vertex from dual connect by level <= (select max(cte.num_vertices) from cte )

完整的查询：

with cte as (
select      1 as road_id,      2 as num_vertices    from dual
union all
select      2 as road_id,   null as num_vertices    from dual
union all
select      3 as road_id,   1000 as num_vertices    from dual
),
numbers as (select level as vertex from dual connect by level <= (select max(cte.num_vertices) from cte ))
select
    *
from
    cte
cross join
     numbers
where    
    vertex <= num_vertices
order by
    road_id,
    vertex

db<>fiddle

Answer 2

一种方法是递归 CTE：

with cte as (
      select      1 as road_id,      2 as num_vertices    from dual
      union all
      select      2 as road_id,   null as num_vertices    from dual
      union all
      select      3 as road_id,   1000 as num_vertices    from dual
     ),
     rcte (road_id, vertex, num_vertices) as (
      select road_id, 1 as vertex, num_vertices
      from cte
      where num_vertices > 0
      union all
      select road_id, vertex + 1, num_vertices
      from rcte
      where vertex < num_vertices
     )
select road_id, vertex
from rcte;

Here 是一个 dbfiddle。