我想从日期中提取月份和日期答案

【问题标题】：I would like to extract the month and day from a date我想从日期中提取月份和日期
【发布时间】：2021-01-07 16:02:27
【问题描述】：

我的代码

SELECT 
    PARSE_DATE('%Y%m%d', CAST(date AS STRING)) AS date, 
    EXTRACT(DAY from date) AS day_of_month,
    EXTRACT(MONTH from date) AS week_of_year,  
    channelGrouping, deviceCategory, sessions, conversions,

我想将列中的日期（格式为 20170101）转换为 2017-01-01。

然后我想从中提取月份和日期。但是我不断收到错误消息：

参数类型的函数 EXTRACT 没有匹配的签名：DATE_TIME_PART FROM INT64。支持的签名：EXTRACT(DATE_TIME_PART FROM DATE); EXTRACT(DATE_TIME_PART FROM TIMESTAMP [AT TIME ZONE STRING]);提取（DATE_TIME_PART FROM DATETIME）； [1:60] 提取（DATE_TIME_PART FROM TIME）

【问题讨论】：

标签： sql google-bigquery

【解决方案1】：

BigQuery 标准 SQL 的以下几个选项 - 避免多次预解析

选项 1

SELECT 
  date, 
  EXTRACT(DAY FROM date) AS day_of_month,
  EXTRACT(WEEK FROM date) AS week_of_year,  
  channelGrouping, deviceCategory, sessions, conversions, 
FROM `project.dataset.table`, 
UNNEST([PARSE_DATE('%Y%m%d', CAST(date AS STRING))]) date

选项 2

SELECT 
  date, 
  EXTRACT(DAY FROM date) AS day_of_month,
  EXTRACT(WEEK FROM date) AS week_of_year,  
  channelGrouping, deviceCategory, sessions, conversions, 
FROM (
  SELECT * REPLACE(PARSE_DATE('%Y%m%d', CAST(date AS STRING)) AS date)  
  FROM `project.dataset.table`
)

【讨论】：

【解决方案2】：

你需要重新解析每个extract()的日期：

SELECT PARSE_DATE('%Y%m%d', CAST(date AS STRING)) as date,
       EXTRACT(DAY from PARSE_DATE('%Y%m%d',
       EXTRACT(MONTH from PARSE_DATE('%Y%m%d', CAST(date AS STRING))) as week_of_year,
       channelGrouping, deviceCategory, sessions, conversions,

但是，如果date 已经是 YYYYMMDD 格式的数字，为什么不直接使用算术函数：

select mod(date, 100) as day,
       mod(floor(date / 100), 100) as month

【讨论】：