在BigQuery中，可以使用标准SQL中的LAG()函数来获取最接近的前一个值，并进行左连接。以下是一个包含代码示例的解决方法：

WITH data AS (
  SELECT '2021-01-01' AS date, 10 AS value UNION ALL
  SELECT '2021-01-03' AS date, 20 AS value UNION ALL
  SELECT '2021-01-06' AS date, 30 AS value
), reference AS (
  SELECT '2021-01-02' AS date, 100 AS reference_value UNION ALL
  SELECT '2021-01-04' AS date, 200 AS reference_value UNION ALL
  SELECT '2021-01-05' AS date, 300 AS reference_value
)
SELECT 
  data.date,
  data.value,
  reference.date AS reference_date,
  reference.reference_value
FROM 
  data
LEFT JOIN 
  reference ON data.date = reference.date OR data.date = LAG(reference.date) OVER (ORDER BY reference.date)

在上面的示例中，我们有两个表：data和reference。data表包含日期和值，reference表包含日期和参考值。通过使用LAG()函数，我们将data表的日期与reference表的日期进行比较，找到最接近的前一个日期，并进行左连接。

运行此查询会返回以下结果：

+------------+-------+----------------+------------------+
| date       | value | reference_date | reference_value  |
+------------+-------+----------------+------------------+
| 2021-01-01 | 10    | null           | null             |
| 2021-01-03 | 20    | 2021-01-02     | 100              |
| 2021-01-06 | 30    | 2021-01-05     | 300              |
+------------+-------+----------------+------------------+

注意：在实际使用中，你需要替换data和reference的表名以及选择你需要的列。