sql查詢基於另一個表中的條目派生列值

Question

我有一個人表字段id,email。

和訂閱表中字段：person_id,category_id。

如果一個人訂閱訂閱表中至少有一個表中的條目。

我想查詢人表中第三列有1或0取決於人是否訂閱。

select p.id, p.email , [is_subscribed] 
from person p 

is_subscribed應該是1，如果在訂閱其他表的條目應爲0

我怎樣才能實現上述？

Answer 1

使用LEFT JOIN上訂閱&計數（在多個預訂的情況下）

SELECT p.id,p.email,IF(COUNT(s.category_id)>0,1,0) as is_subscribed 
FROM 
person as p 
LEFT JOIN subscription as s ON s.person_id=p.id 
GROUP BY p.id 

Answer 2

select distinct p.id, p.email, ifnull(sign(subscription.id),0) as is_subscribed 
    from 
     person 
      left join subscriptions 
      on person.id = subscriptions.person_id 

Answer 3

SELECT p.`id` , p.`email` , IF(COUNT(s.`id`)>0,1,0) AS count 
FROM `person` p 
LEFT JOIN `subscription` s ON s.`person_id` = p.`id` 

Answer 4

select distinct person.id, person.email, cast((select count(id) from sub where id=1) as bit) from tab, sub where tab.id=1;

這是一個MS SQL Server查詢。對mysql進行必要的更改。