Question

I’m a little stuck with an sql query. I’ve searched everywhere but to no avail so was hoping somebody here could help.

I have 2 tables.

Table 1 is a list of notifications with the following columns

• notification_id (this is the auto incremented primary key)
• notification (This is the notification message)
• from_id (This is the user_id of the person sending the notification)

eg.

|  notification_id  |  notification  |  from_id  |
|       25          |  this message  |    7      |
|       26          |  that message  |    8      |

Table 2 shows which users have read each notification. It has the following columns

• notification_id (this is the id of the relevant notification)
• user_id (This is the user id of the person who has read the notification)

eg.

| notification_id   |   user_id   |
|     25            |      1      |
|     25            |      2      |
|     25            |      3      |
|     26            |      2      |

The tables are joined on notification_id and we have a one to many relationship as many users will read each notification.

What I am trying to do is query the database to return all the notifications that the particular user has not already read. In other words I want the data from table one but only if table 2 doesn’t have an entry where user_id is (My user_id x) for the corresponding notification_id.

So if my user_id is 4 then I get the data for both notification_id 25 and 26 as user_id 4 is not listed in table 2 for either of those

If my user_id is 2 I don’t get any data as user_id 2 is listed in table 2 as having read both notification_id 25 and 26

If my user_id is 1 I get the data for notification_id 26 but not 25

I originally thought I could use this

SELECT
    notification_id, notification, from_id 
FROM table_1 
INNER JOIN table 2 
    ON table_1.notification_id = table2.notification_id
WHERE table2.user_id != X

But this is actually a long way off what I need and I’ve reached a bit of a dead end.

Thanks

Answer 1

I would do a left join and look for empty joins.

SELECT notification_id, notification, from_id FROM table_1
LEFT JOIN table_2 ON table_2.notification_id = table_1.notification_id AND table_2.user_id = x
WHERE table_2.notification_id IS NULL