Question

该查询仅在存在 :: DATE 时有效。

 -- Wrap the query you wrote in a CTE named reg_dates
    WITH reg_dates AS (
      SELECT
        user_id,
        MIN(order_date) AS reg_date
      FROM orders
      GROUP BY user_id)
    
    SELECT
      -- Count the unique user IDs by registration month
      DATE_TRUNC('month', reg_date) :: DATE AS delivr_month,
      COUNT(DISTINCT user_id) AS regs
    FROM reg_dates
    GROUP BY delivr_month
    ORDER BY delivr_month ASC;

为什么需要这样做？当我运行下面的查询时，没有 :: DATE，它不起作用。

-- Wrap the query you wrote in a CTE named reg_dates
WITH reg_dates AS (
  SELECT
    user_id,
    MIN(order_date) AS reg_date
  FROM orders
  GROUP BY user_id)

SELECT
  -- Count the unique user IDs by registration month
  DATE_TRUNC('month', reg_date) AS delivr_month,
  COUNT(DISTINCT user_id) AS regs
FROM reg_dates
GROUP BY delivr_month
ORDER BY delivr_month ASC;

Answer 1

您的 RDBMS 很可能是 PostgreSQL，在您的情况下，:: 将日期类型转换为日期，此外，:: 表示为 CAST(expression AS type)。

同样，

CAST (DATE_TRUNC('month', reg_date) AS DATE) AS delivr_month

Answer 2

“不起作用”是什么意思？请注意，PostgreSQL 中的 date_trunc 返回一个 datetime。因此，如果您的查询需要一个日期，这就是您需要 ::date 的原因。

DATE_TRUNC 带 :: 和不带

2 个答案: