子查询选择内部 where 子句的外部值

Question

用户可以在订阅表中拥有多条记录。

我想要做的是返回他们的名字、姓氏、电子邮件、开始日期（他们的第一次订阅，从订阅顺序中选择 start_date 按 start_date asc 限制 1，但我需要该特定用户）

// users
id
first_name
last_name
email

// subscriptions
id
email
start_date (TIMESTAMP)
end_date (TIMESTAMP)
status

我认为这会起作用，但它似乎没有：

select 
    distinct(users.email), status, first_name, last_name,
    (select start_date from subscriptions where subscriptions.email = users.email order by start_date asc limit 1) as start_date 
from 
    subscriptions sub 
join 
    users u on sub.email = u.email
order by 
    sub.end_date desc

这会为每个人返回相同的 start_date，因为它可能会提取匹配的第一个。

SQL 修改架构：http://sqlfiddle.com/#!9/245c05/5

Answer 1

这个查询：

select s.*
from subscriptions s
where s.start_date = (select min(start_date) from subscriptions where email = s.email) 

返回每个用户第一次订阅的行。
加入users:

select u.*, t.status, t.start_date
from users u 
left join (
  select s.*
  from subscriptions s
  where s.start_date = (select min(start_date) from subscriptions where email = s.email)  
) t on t.email = u.email  

请参阅demo。
结果：

| id  | email          | first_name | last_name | status   | start_date          |
| --- | -------------- | ---------- | --------- | -------- | ------------------- |
| 1   | john@aol.com   | John       | Smith     | active   | 2018-02-12 23:34:02 |
| 2   | jim@aol.com    | Jim        | Smith     | canceled | 2016-03-02 23:34:02 |
| 3   | jerry@aol.com  | Jerry      | Smith     | active   | 2017-12-12 23:34:02 |
| 4   | jackie@aol.com | Jackie     | Smith     | active   | 2018-05-22 23:34:02 |