例如,我尝试过这样做: ifnull(FIRST_VALUE(date) OVER (PARTITION BY client_ID ORDER BY date ASC),0) AS first_conversion_date, 但得到以下错误: No matching signature for function IFNULL for argument types: DATE, INT64.Supported signature: IFNULL(ANY, ANY) at [26:9] Learn More about BigQuery SQ