为何该SQL查询过滤周一数据时返回0行？请求排查

你的问题核心原因是：to_char(install_date, 'Day')返回的星期名字符串带有末尾填充空格（比如PostgreSQL中该格式会自动补空格，让所有星期名长度统一，Monday会变成Monday ），直接用weekday = 'Monday'自然匹配不到。

以下是几种可行的解决办法：

• 方法一：用TRIM()去除空格
  修改CTE中的weekday字段定义，提前去掉字符串两端的空格：

  WITH cte1
       AS (SELECT o.order_id,
                  p.price * o.quantity          AS total_spending,
                  TRIM(to_char (install_date, 'Day')) AS weekday
           FROM   orders AS o
                  LEFT JOIN parts AS p using (part_id)
                  LEFT JOIN installs AS i
                         ON o.order_id = i.order_id)
  SELECT *
  FROM   cte1
  WHERE  weekday = 'Monday' 
  

• 方法二：使用FMDay格式符（PostgreSQL专属）
  FM前缀可以去掉格式输出中的填充空格，直接返回无空格的星期全名：

  WITH cte1
       AS (SELECT o.order_id,
                  p.price * o.quantity          AS total_spending,
                  to_char (install_date, 'FMDay') AS weekday
           FROM   orders AS o
                  LEFT JOIN parts AS p using (part_id)
                  LEFT JOIN installs AS i
                         ON o.order_id = i.order_id)
  SELECT *
  FROM   cte1
  WHERE  weekday = 'Monday' 
  

• 方法三：在WHERE子句中直接处理（不推荐）
  如果不想修改CTE，也可以在过滤条件中对weekday做修剪：

  WITH cte1
       AS (SELECT o.order_id,
                  p.price * o.quantity          AS total_spending,
                  to_char (install_date, 'Day') AS weekday
           FROM   orders AS o
                  LEFT JOIN parts AS p using (part_id)
                  LEFT JOIN installs AS i
                         ON o.order_id = i.order_id)
  SELECT *
  FROM   cte1
  WHERE  TRIM(weekday) = 'Monday' 
  

另外补充：你用了LEFT JOIN installs，如果部分订单没有对应的安装记录，install_date会为NULL，对应的weekday也会是NULL，这部分数据会被WHERE条件过滤。如果需要包含这类订单，可调整条件为weekday = 'Monday' OR weekday IS NULL，但根据你描述的场景，核心问题还是空格匹配的问题。

内容的提问来源于stack exchange，提问作者Jahnavi Patel