如何选择集合的第一个元素作为查询的列

表 t 有两列：a 和 b。 a是整数，b是集合。

我想为每一行选择 a 和集合 b 的第一个值。

我试过了，但没用

WITH
    s (a, b)
    AS
        (SELECT 1, 'ff' FROM DUAL
         UNION ALL
         SELECT 1, 'ee' FROM DUAL
         UNION ALL
         SELECT 1, 'ee' FROM DUAL
         UNION ALL
         SELECT 2, 'ee' FROM DUAL),
    t (a, b)
    AS
        (  SELECT s.a, COLLECT (s.b)
             FROM s
         group BY s.a)
select t.a, t.b.first()
from t

[错误]执行（42：16）：ORA-00904：“T”。“B”。“FIRST”：无效标识符

有办法做到这一点吗？

此查询在不首先使用集合方法的情况下执行相同的操作。

WITH
    s (a, b)
    AS
        (SELECT 1, 'ff' FROM DUAL
         UNION ALL
         SELECT 1, 'ee' FROM DUAL
         UNION ALL
         SELECT 1, 'ee' FROM DUAL
         UNION ALL
         SELECT 2, 'ee' FROM DUAL),
    t (a, b)
    AS
        (  SELECT s.a, COLLECT (s.b)
             FROM s
         GROUP BY s.a),
    v (a, b)
    AS
        (SELECT t.a, tb.b
           FROM t
                OUTER APPLY (SELECT x.COLUMN_VALUE     b
                               FROM TABLE (t.b) x
                              FETCH FIRST 1 ROW ONLY) tb)
SELECT *
  FROM v;