Question

我正在使用Psql，需要通过创建一个数据透视表从名为activities的表中获取值。

目前我有这样的查询，它会生成一个包含first_name|email|signup_hash列的表：

SELECT COALESCE(profile.first_name, signup_hash.first_name) AS first_name,
       users.email,
       signup_hash.hash_id,

FROM signup_hash
LEFT OUTER JOIN users ON users.id = signup_hash.user_id
LEFT OUTER JOIN profiles ON profiles.user_id = users.id
INNER JOIN memberships ON memberships.hash_id = signup_hash.hash_id
WHERE memberships.member_id =?

activities表包含以下字段：user_id|activity_name|date_completed|date_created

每个用户的活动都包含activity_name“first_login”和“profile_complete”（例如，activity.name =“first_login”和activity.name =“profile_complete”）。

我如何加入活动表，以便最终查询生成一个包含列的表：first_name|email|first_login|profile_complete|signup_hash？

修改 1）对于特定用户，没有多行activity_name为“first_login”和“profile_complete” 2）列first_login＆amp;列应该是什么值？ profile_complete包含在最终结果中？如果值存在则应显示date_completed，否则显示“N / A”

Answer 1

您可以按如下方式再使用2个LEFT OUTER JOIN，并使用CASE在结果中选择所需的表达式。请注意，我在此处将date_completed视为date列。如果是字符列，则不需要TO_CHAR。

    SELECT COALESCE(profile.first_name, signup_hash.first_name) AS first_name,
       users.email,
       CASE
           WHEN f_login.user_id IS NULL THEN 'N/A'
           ELSE TO_CHAR(date_completed)
       END first_login,
       CASE
           WHEN p_complete.user_id IS NULL THEN 'N/A'
           ELSE TO_CHAR(date_completed)
       END profile_complete,
       signup_hash.hash_id
FROM signup_hash
LEFT OUTER JOIN users ON users.id = signup_hash.user_id
LEFT OUTER JOIN profiles ON profiles.user_id = users.id
INNER JOIN memberships ON memberships.hash_id = signup_hash.hash_id
LEFT OUTER JOIN
  (SELECT user_id,
          date_completed
   FROM activities
   WHERE activity_name = 'first_login') f_login ON f_login.user_id = signup_hash.user_id
LEFT OUTER JOIN
  (SELECT user_id,
          date_completed
   FROM activities
   WHERE activity_name = 'profile_complete') p_complete ON p_complete.user_id = signup_hash.user_id
WHERE memberships.member_id = ?

在sql

1 个答案: