mysql full join not working correctly - mysql

Question

I am using the following mysql to look at user subscriptions. I need to get all the users and in one column show if they have an active subscription or not
This is what I have tried:
SELECT `general`.`exchange`.`email`,substring_index(`general`.`exchange`.`name`, " ", 1) as name, `courses`.`courseSubscriptions`.`expiryTimestamp`
FROM `general`.`exchange`
LEFT JOIN `courses`.`courseSubscriptions`
ON `courses`.`courseSubscriptions`.`memberID` = `general`.`exchange`.`id`
WHERE (`courses`.`courseSubscriptions`.memberID = `general`.`exchange`.`id`)
AND (`courses`.`courseSubscriptions`.expiryTimestamp > 1443975741)
The problem is that it is only returning users who have a subscription. I need it to return all users, and show in a column who has a subscription or not
How can I do this?

Answer 1

Your where clause is turning the left join into an inner join. Also, table aliases would make the query easier to write and to read:
SELECT e.`email`, substring_index(e.`name`, ' ', 1) as name, cs.`expiryTimestamp`
FROM `general`.`exchange` e LEFT JOIN
`courses`.`courseSubscriptions` cs
ON cs.`memberID` = e.`id` AND (cs.expiryTimestamp > 1443975741) ;
The solution to the LEFT JOIN problem is to move the condition on the second table into the ON clause. Also, you don't need to repeat the join conditions.
If you want a flag, you can add that:
SELECT e.`email`, substring_index(e.`name`, ' ', 1) as name, cs.`expiryTimestamp`,
(cs.`expiryTimestamp` is not null) as isActiveFlag
FROM `general`.`exchange` e LEFT JOIN
`courses`.`courseSubscriptions` cs
ON cs.`memberID` = e.`id` AND (cs.expiryTimestamp > 1443975741) ;

Answer 2

You've repeated the join condition in the where clause. Since null will never return true in an equality check, this effectively removes the users without any courses and turns your outer join to an inner join. Similarly, the condition on the course's time should also be moved to the join clause:
SELECT `general`.`exchange`.`email`,
SUBSTRING_INDEX(`general`.`exchange`.`name`, " ", 1) AS name,
`courses`.`courseSubscriptions`.`expiryTimestamp`
FROM `general`.`exchange`
LEFT JOIN `courses`.`courseSubscriptions` ON
`courses`.`courseSubscriptions`.`memberID` =
`general`.`exchange`.`id` AND
`courses`.`courseSubscriptions`.expiryTimestamp > 1443975741