SQL 中的 LISTAGG 返回具有空值的行

Question

我有 2 个表 A 和 B，B 与 A 有外键关系，即 (b.detail_id = a.id) 我想对 B 中的一列应用 LISTAGG 查询。

SELECT LISTAGG(DISTINCT b.delivery_cadence, ',') WITHIN GROUP (ORDER BY b.delivery_cadence) 
delivery_cadence, a.id FROM A a, B b WHERE b.detail_id = a.id AND a.id = 1236565;

上面的查询返回给我一个所有值都为空的行，但我不想要任何行。我怎样才能做到这一点？ 如果不可能有任何替代解决方案。

A表中不存在a.id = 1236565。

Answer 1

只需添加having count(b.delivery_cadence) > 0 例如

SELECT LISTAGG(DISTINCT b.delivery_cadence, ',') WITHIN GROUP (ORDER BY b.delivery_cadence) 
delivery_cadence, a.id FROM A a, B b WHERE b.detail_id = a.id AND a.id = 1236565
HAVING COUNT(b.delivery_cadence) > 0

Answer 2

假设您有这样的表格：

create table a(id) as
(
    select 1 from dual union all
    select 2 from dual
);
create table b(detail_id, delivery_cadence) as
(
    select 1, 'x' from dual union all
    select 1, 'A' from dual union all
    select 2, 'y' from dual
);

如果我理解得很好，你需要（在 ANSI 连接语法中）：

SQL> SELECT LISTAGG(DISTINCT b.delivery_cadence, ',') WITHIN GROUP (ORDER BY b.delivery_cadence) delivery_cadence,
  2          a.id
  3  FROM A
  4      inner join B ON b.detail_id = a.id
  5  where a.id = 1236565
  6  group by a.id;

no rows selected

对于表中存在的ID 值，您会得到：

SQL> SELECT LISTAGG(DISTINCT b.delivery_cadence, ',') WITHIN GROUP (ORDER BY b.delivery_cadence) delivery_cadence,
  2          a.id
  3  FROM A
  4      inner join B ON b.detail_id = a.id
  5  where a.id = 1
  6  group by a.id;

DELIVERY_CADENCE         ID
---------------- ----------
A,x                       1

1 row selected.