BigQuery 取消嵌套到新列而不是新行

Question

我有一个 BigQuery 表，其中包含嵌套和重复的列，我无法取消嵌套到列而不是行。

这就是表格的样子：

  {
    "dateTime": "Tue Mar 01 2022 11:11:11 GMT-0800 (Pacific Standard Time)",
    "Field1": "123456",
    "Field2": true,
    "Matches": [
      {
        "ID": "abc123",
        "FieldA": "asdfljadsf",
        "FieldB": "0.99"
      },
      {
        "ID": "def456",
        "FieldA": "sdgfhgdkghj",
        "FieldB": "0.92"
      },
      {
        "ID": "ghi789",
        "FieldA": "tfgjyrtjy",
        "FieldB": "0.64"
      }
    ]
  },

我想要的是将每个嵌套字段作为单独的列返回一个表，这样我就有了一个干净的 dataframe 可以使用：

  {
    "dateTime": "Tue Mar 01 2022 11:11:11 GMT-0800 (Pacific Standard Time)",
    "Field1": "123456",
    "Field2": true,
    "Matches_1_ID": "abc123",
    "Matches_1_FieldA": "asdfljadsf",
    "Matches_1_FieldB": "0.99", 
    "Matches_2_ID": "def456",
    "Matches_2_FieldA": "sdgfhgdkghj",
    "Matches_2_FieldB": "0.92",
    "Matches_3_ID": "ghi789",
    "Matches_3_FieldA": "tfgjyrtjy",
    "Matches_3_FieldB": "0.64"
  },

我尝试如下使用 UNNEST，但是它创建的新行只有一组附加列，所以不是我要找的。

SELECT * 
FROM table,
UNNEST(Matches) as items

有什么解决办法吗？ 先感谢您。

Answer 1

考虑以下方法

select * from (
  select t.* except(Matches), match.*, offset + 1 as offset 
  from your_table t, t.Matches match with offset
)
pivot (min(ID) as id, min(FieldA) as FieldA, min(FieldB) as FieldB for offset in (1,2,3))             

如果应用于 y 中的示例数据，我们的问题 - output 是

如果没有匹配项，则整行不包含在 output 表中，而我想保留该记录及其所具有的字段。 我怎样才能修改以保留所有记录？

使用 left join 代替，如下所示

select * from (
  select t.* except(Matches), match.*, offset + 1 as offset 
  from your_table t left join t.Matches match with offset
)
pivot (min(ID) as id, min(FieldA) as FieldA, min(FieldB) as FieldB for offset in (1,2,3))