mysql - 如何获取一对多连接中 NULL 记录的数量

Question

我有一个帐户和计划表，其中用户的帐户在给定时间可以没有与其关联的单个/多个或非事件计划。

帐户表:

+---------+-------------+---+
|   ID    | account_id  | ..|
+---------+-------------+---+
|   1     |   111111    |   |
|   2     |   222222    |   |
|   3     |   333333    |   |
|   4     |   444444    |   |
+---------+-------------+---+

计划表:

+----+--------------------+----------------+-----------------+---------------------+---------------------+
| id |     account_id     | attribute_key  | attribute_value | start_date          | end_date            |
+----+--------------------+----------------+-----------------+---------------------+---------------------+
|  1 |             111111 |      RPC       | AB              | 2011-10-01 00:00:00 | NULL                |
|  2 |             111111 |      RPC       | CND             | 2011-10-01 00:00:00 | NULL                |
|  3 |             222222 |      RPC       | IA              | 2011-10-01 00:00:00 | 2015-04-01 00:00:00 |
|  4 |             222222 |      RPC       | CND             | 2011-10-01 00:00:00 | 2015-04-01 00:00:00 |
|  5 |             333333 |      RPC       | IA              | 2011-10-01 00:00:00 | 2015-04-01 00:00:00 |
|  6 |             333333 |      RPC       | CND             | 2011-10-01 00:00:00 | NULL                |
+----+--------------------+----------------+-----------------+---------------------+---------------------+

如果 end_date 为 NULL，则关联计划被视为有效。 我正在尝试提出一个查询，该查询将返回仅与非事件计划关联的帐户列表或计数。根据上面的内容，它将是 222222，因为它有两个与之关联的计划，但都被标记为非事件状态。:

+----+--------------------+----------------+-----------------+---------------------+---------------------+
| id |     account_id     | attribute_key  | attribute_value | start_date          | end_date            |
+----+--------------------+----------------+-----------------+---------------------+---------------------+
|  3 |             222222 |      RPC       | IA              | 2011-10-01 00:00:00 | 2015-04-01 00:00:00 |
|  4 |             222222 |      RPC       | CND             | 2011-10-01 00:00:00 | 2015-04-01 00:00:00 |
+----+--------------------+----------------+-----------------+---------------------+---------------------+

Answer 1

在派生表(子查询)中，您可以获得仅包含“非事件计划”的 account_id 值列表:

SELECT account_id 
FROM plan 
GROUP BY account_id 
HAVING COUNT(end_date IS NULL) = 0 /* No plan with null end date */

现在，您可以使用此子查询连接回 plan 表，以获取该 account_id 的所有计划:

SELECT p.* 
FROM plan AS p 
JOIN (
       SELECT account_id 
       FROM plan 
       GROUP BY account_id 
       HAVING COUNT(end_date IS NULL) = 0
     ) AS dt ON dt.account_id = p.account_id