Left Join of two tables based on a range of dates - sql

Question

I am a newbie to SQL coding and am trying to figure out how to create a LEFT JOIN statement based on a date range. The database is analytics from a smartphone app that sends messages to users. The two tables are messageLog (which describes the messages sent to each user) and messageOpenLog (which describes the messages that are opened). Both tables are linked to the message table, but not to each other. To complicate matters, there are a couple other rules we have developed on when messages are able to be sent:
If a message is not opened within 7 days, the message can be resent on day 8.
If a message is opened, then the message can be resent within 60 days.
So, what I want to do is join the two tables together based on the following pseudocode (as I have no idea where to start with actual code):
LEFT JOIN
If (messageOpenLog.DateOpened is within 7 days of messageLog.DateSent)
and messageLog.message_id = messageOpenLog.message_id and
messageLog.user_id = messageOpenLog.user_id
Note: the date format is yyyy-mm-dd hh:mm:ss in both tables.
Any help you can provide would be greatly appreciated.

Answer 1

I am unable to comment on shn's answer, but there is a chance that the user has never opened the message and a messageOpenLog record has not been created. In that case you could add a messageOpenLog.message_id is null to the where clause and get those unopened messages with no corresponding messageOpenLog record as well.

Answer 2

I would suggest:
messagelog ml LEFT JOIN
messageOpenLog mol
ON mol.message_id = ml.message_id AND
mol.user_id = ml.user_id AND
mol.DateOpened >= ml.DateSent AND -- probably not needed
mol.DateOpened < ml.DateSent + interval '7 day'
Note that date arithmetic varies a lot among databases. The exact syntax for adding seven days may be a bit different in your database.

Answer 3

From what I understand from your question and the query below, you are finding all the individual messages that have a time difference of longer than 7 days between their send date and open date.
To do the time difference I would recommend using the DATEDIFF() function that is built into SQL (you may need to format the timestamps into date format with DATE()).
Since the two tables are not directly related you could do something like this:
SELECT
messageOpenLog.*,
messageLog.*
FROM
messageLog LEFT JOIN messageOpenLog ON messageLog.message_id=messageOpenLog.message_id
WHERE
messageLog.user_id = messageOpenLog.user_id AND
DATEDIFF(
day,
DATE(messageLog.timestamp),
DATE(messageOpenLog.timestamp)
) > 7
The structure of this query is dependent on the construction of your tables.
Notice I used the .timestamp column but in your tables this may be named different.
Also I'm not sure if this is actually what you want; if you want to see if the message is more than 7 days old, a different query is required.
Assuming that there is only one messageSent row, this query will get all of the messageOpen rows for the same message that are more than 7 days old.
It's very difficult to give you an exact query based on the information that was presented, such as the potential number of rows with the same message_id, as #amac mentions, there could also be cases where one of the tables has no rows with a certain message_id.