问题
I want to implement an alert messaging system, in a website, that will show a message to either multiple users, multiple groups (different DB representation), or globally to all users.
What would a good database relationship for this behavior look like?
回答1:
This is where I would start based on the information you've given...
A table for the alert messages to be sent out...
alerts
id unsigned int(P)
message text
+----+---------------------+
| id | message |
+----+---------------------+
| 1 | The sky is falling! |
| 2 | Some other message |
| .. | ................... |
+----+---------------------+
A table to hold information about all the groups...
groups
id unsigned int(P)
name varchar(20)
+----+---------+
| id | name |
+----+---------+
| 1 | Group A |
| 2 | Group B |
| .. | ....... |
+----+---------+
A table to associate groups and alerts. Each column is a foreign key to it's respective table and together they form a primary key - this will make sure no group is every associated with the same alert more than once. In my example data we see that Group A is signed up for both "The sky is falling!" and "Some other message" messages while Group B is only signed up for "Some other message" alerts.
groups_alerts
alert_id unsigned int(F alerts.id)--\_(P)
group_id unsigned int(F groups.id)--/
+----------+----------+
| alert_id | group_id |
+----------+----------+
| 1 | 1 |
| 1 | 2 |
| 2 | 2 |
| ........ | ........ |
+----------+----------+
A table for user information...
users
id unsigned int(P)
username varchar(32)
password varbinary(255)
...
+----+----------+----------+-----+
| id | username | password | ... |
+----+----------+----------+-----+
| 1 | bob | ******** | ... |
| 2 | mary | ******** | ... |
| 3 | john | ******** | ... |
| .. | ........ | ........ | ... |
+----+----------+----------+-----+
A table that associates users with alerts. The keys follow the same logic as those in the groups_alerts table. In my example data we see that john is signed up for the "The sky is falling!" message.
users_alerts
user_id unsigned int(F users.id)---\_(P)
alert_id unsigned int(F alerts.id)--/
+---------+----------+
| user_id | alert_id |
+---------+----------+
| 3 | 1 |
| ....... | ........ |
+---------+----------+
And finally we need to associate users with groups. The keys follow the same logic as those in the groups_alerts table. In my example data we see that bob is a member of Group A and mary is a member of Group B.
users_groups
user_id unsigned int(F users.id)
group_id unsigned int(F groups.id)
+---------+----------+
| user_id | group_id |
+---------+----------+
| 1 | 1 |
| 2 | 2 |
| ....... | ........ |
+---------+----------+
来源：https://stackoverflow.com/questions/21333875/database-messaging-for-multiple-model-types-user-group-global