multiple child for one parent in mysql

i have a table of users and a table of activities.
each user can be related to a few activities

when i enter an activity, i need display the user list and vice versa
how can i make that connection?
if i seperate the ids by commas, i won't be able to make a selection by id

 

You would use 2 tables:

user_table
user_id
other_columns

activities_table
activity_id
user_id
activity_name

You can then add as many activities as you want for each user. You can do a Join to connect the activities of each user.

 

i need many users to be related to the same activity id...

 

I see. It would be a little different then.

activities_table
activity_id
activity_name

user_table
user_id
other_columns

user_activities
ua_id
activity_id
user_id

 

thought there might be an easier way, without using 3 tables.
thank you.

 

You can use a JOIN to get this information very easily. It looks more complicated, but storing as a csv prevents MySQL from being able to use the data.

To get activities:
SELECT activities_table.activity_name FROM activities_table
LEFT JOIN user_activities ON activities_table.activity_id = user_activities.activity_id
WHERE user_activities.user_id = '';

To get users:
SELECT user_table.user_name FROM user_table
LEFT JOIN user_activities ON user_table.user_id = user_activities.user_id
WHERE user_activities.activity_id = '';