Join tables when one table is empty - mysql

Question

I have three tables which I need to join: User, Notification and UserNotification. The latter is just a cross reference table between User and Notificaion with columns UserID (fk to User) and LastReadNotificationID (fk to Notification). Table UserNotification should contain a reference to the last read Notification in a user-notification feature in a web app. So if we have two records in Notification with ID 1 and 2, and one record in UserNotification with a fk to Notification = 1, it means that the user has NOT read the last created notification which I want to display at the next login.
Now, I need to select all columns from the User table at login and add another column (Notify) to the resultset. Notify should be a boolean that should be false if:
Notification is empty
or Notification contains a record with eg ID = 10 AND UserNotification does have the corresponding foreign key.
Notify should be true if:
Notification contains a record AND UserNotification is empty.
or Notification contains a record with eg ID = 10 BUT UserNotification does NOT have the corresponding foreign key.
The problem is that I can't write a query that meets all the above requirements. The query I have at the moment works except when Notification is empty (and thus is UserNotification). In this case my query returns Notify = true;
If have tried many different ways to solve (left joins, right joins, if, case when, ifnull etc) this but I'm stuck. Please help.
The query I use now:
SELECT ID, FirstName, LastName, Email, Password, Roles, LastLoginTime, LoginCount, Active,
(SELECT IFNULL((SELECT 0 FROM UserNotification UN, User U
WHERE UN.UserId = U.ID AND U.Email = :email
AND UN.LastReadNotificationID <=> (SELECT MAX(ID) FROM Notification WHERE Display = 1)), 1)) AS Notify
FROM User WHERE Email = :email;
The 3 tables:
CREATE TABLE `User` (
`ID` bigint(20) NOT NULL AUTO_INCREMENT,
`FirstName` varchar(50) CHARACTER SET utf8 DEFAULT NULL,
`LastName` varchar(50) CHARACTER SET utf8 DEFAULT NULL,
`Email` varchar(50) CHARACTER SET utf8 DEFAULT NULL,
`Password` varchar(200) CHARACTER SET utf8 DEFAULT NULL,
`Roles` varchar(200) CHARACTER SET utf8 DEFAULT NULL,
`LastLoginTime` varchar(50) CHARACTER SET utf8 DEFAULT NULL,
`LoginCount` int(11) DEFAULT NULL,
`Active` bit(1) NOT NULL DEFAULT b'1',
PRIMARY KEY (`ID`),
UNIQUE KEY `Email` (`Email`)
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=latin1;
CREATE TABLE `UserNotification` (
`UserID` bigint(20) NOT NULL,
`LastReadNotificationID` int(11) NOT NULL,
UNIQUE KEY `UserID_UNIQUE` (`UserID`),
KEY `fk_UserNotification_Notification_ID` (`LastReadNotificationID`),
CONSTRAINT `fk_UserNotification_Notification_ID` FOREIGN KEY (`LastReadNotificationID`) REFERENCES `Notification` (`ID`) ON DELETE CASCADE,
CONSTRAINT `fk_UserNotification_User_ID` FOREIGN KEY (`UserID`) REFERENCES `User` (`ID`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
CREATE TABLE `Notification` (
`ID` int(11) NOT NULL AUTO_INCREMENT,
`Text` text NOT NULL,
`Created` timestamp NOT NULL,
`UserID` bigint(20) NOT NULL,
`Display` bit(1) DEFAULT NULL,
PRIMARY KEY (`ID`),
KEY `fk_Notification_User_ID` (`UserID`),
CONSTRAINT `fk_Notification_User_ID` FOREIGN KEY (`UserID`) REFERENCES `User` (`ID`) ON DELETE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=latin1;

Answer 1

The problem with your attempts probably boils down to one basic issue: you want to chain the three tables together with left joins (A -> B -> C) and yet you also want to be able to infer whether there are any other rows in the table that fall outside of the join logic and the pairs of IDs.
B and C can both be empty which is why it's natural to approach this with left joins. (B's "emptiness" is per user, of course.) The problem is that B sits in the middle and it can be empty even if C is not. But when B is empty you can't determine anything conclusive about C just from the results of the join.
select
u.ID as UserID, FirstName, LastName, Email, Password,
Roles, LastLoginTime, LoginCount, Active,
case
when max_n.IsEmpty = 1 or un.LastReadNotification = max_n.ID then 0
-- the following is equivalent and eliminates the IsEmpty flag entirely
-- when coalesce(max_n.ID, 0) = coalesce(un.LastReadNotification, 0) then 1
else 1 -- isn't it sufficient to just return 1 at this point?
-- when un.LastReadNotification is null then 1 -- notification wasn't empty btw
-- when un.LastReadNotification < agg_n.MaxID then 1 -- can't be greater, right?
end as Notify
from
User u
left outer join UserNotification un
on un.UserID = u.ID
cross join (
select
case when max(ID) is not null then 0 else 1 end as IsEmpty,
max(ID) as ID
from Notification
) max_n
You may prefer a subquery over the cross join:
select
u.ID as UserID, FirstName, LastName, Email, Password,
Roles, LastLoginTime, LoginCount, Active,
case
when coalesce((select max(ID) from Notification), 0)
= coalesce(LastReadNotification, 0)
else 1
end as Notify
from User u left outer join UserNotification un on un.UserID = u.ID
Ultimately all you really needed to know is whether there is a notification with ID greater than the ID last seen by each user. Grabbing just the highest notification ID is enough information to let you make that determination across the board.
Here's another thought though: would it possibly be easier to just have LastReadNotification in the user table with an initial value of 0 along with a dummy row with ID 0 in the "empty" notification table? Essentially you remove the need to know anything about emptiness at all. If you implemented the 0 row you'd just end up with something like this:
select
ID as UserID, FirstName, LastName, Email, Password,
Roles, LastLoginTime, LoginCount, Active,
case when exists (
select 1
from Notification n
where n.ID > u.LastReadNotification
) then 1 else 0 end as NotifyA,
(
select sign(count(*))
from Notification n
where n.ID > u.LastReadNotification
) as NotifyB
from User

Answer 2

Please expand on your desired results. Do you want the actual last entered notification that the user has not read only? Or do you just want to know if you should notify the user?
SELECT
u.Id
,u.FirstName
,u.LastName
,u.Email
,u.Password
,u.Roles
,u.LastLoginTime
,u.LoginCount
,u.Active
,CASE WHEN un.UserId IS NULL THEN 1 Notify
FROM
User u
CROSS JOIN Notification n
LEFT JOIN UserNotification un
ON u.ID = un.UserId
AND n.Id = un.LastReadNotificationId
WHERE
U.Email = :email
AND n.Display = 1
;
This query will give you every combination possible for Users to Notifications and then join the UserNotifications table if a record exists. It will specify to notify but for all Notifications that the user has not read. so user will be repeated by all Notifications.
You can then limit to notify and specify the latest Notification the user has not read by using a case statement and group by.
SELECT
u.Id
,u.FirstName
,u.LastName
,u.Email
,u.Password
,u.Roles
,u.LastLoginTime
,u.LoginCount
,u.Active
,CASE
WHEN
SUM(CASE
WHEN un.UserId IS NULL THEN 1
ELSE 0
END) > 0 THEN 1
ELSE 0
END AS Notify
,MAX(CASE WHEN un.UserId IS NULL THEN n.ID END) AS NextNotification
FROM
User u
CROSS JOIN Notification n
LEFT JOIN UserNotification un
ON u.ID = un.UserId
AND n.Id = un.LastReadNotificationId
WHERE
U.Email = :email
AND n.Display = 1
GROUP BY
u.Id
,u.FirstName
,u.LastName
,u.Email
,u.Password
,u.Roles
,u.LastLoginTime
,u.LoginCount
,u.Active
If the user has read all notification the NextNotifcation will be NULL as MAX ignores null values and the case statement doesn't specify and else therefore the value would be null.
If you just want to know if you should notify the user just ignore/remove the NextNotifcation column.
Another way to do this which would be similar to the way you were proceeding would be:
SELECT
u.Id
,u.FirstName
,u.LastName
,u.Email
,u.Password
,u.Roles
,u.LastLoginTime
,u.LoginCount
,u.Active
,CASE
WHEN EXISTS
(SELECT
*
FROM
Notifications n
LEFT JOIN UserNotifications un
ON n.Id = un.LastReadNotificationId
AND un.UserId = u.Id
WHERE
n.Display = 1
)
THEN 1
ELSE 0
END AS Notify
FROM
Users u
WHERE
u.Email = :email
;
Normally I don't like putting a sub select in a column definition but looking at your datatype of BIGINT for users it may actually be better performance to use EXISTS and not the cross join.