SQL查询基于另一个表中的条目派生列值

Question

I have a person table with fields id , email .

And a subscription table with fields: person_id , category_id .

If a person is subscribed subscription table will have at-least one entry in the table.

I want to query person table with the 3rd column having 1 or 0 depending on whether person is subscribed or not.

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

is_subscribed should be 1 if there is an entry in the subscription table else should be 0.

How can I achieve the above ?

Answer 1

Using a LEFT JOIN on subscription & a count (in a case of several subscriptions)

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;

This is an MS SQL server query. Make necessary changes for mysql.