Questions regarding phpMyAdmin Table Relations and DB Structure - mysql

Question

I'm creating a website for use by a Youth Organisation to organize events, by providing a place for upcoming events to be listed and to be signed up to by members. In my database I have a table full of tags that can be assigned to events, quite like this website where you can tag your questions. I have another table to store the information about the events, for example title, description, requirements, date, etc.
I want to connect these databases up, so that when an event is made they are assigned a primary tag and an infinite amount of secondary tags from those in the tag table. Currently, I have a linking table that has a field for the event ID, the tag ID and whether the tag is the primary tag or not, however as I have had to set the fields to unique to allow me to create a relation I cannot store enter multiple event or tag IDs.
My question is, what is the the best way for me to structure my database for the functionality described above? Further more, if what I am doing correctly is correct, then how can I link the tables without either field in the linking table being a primary or unique key?
tblEvents
tblTags

Answer 1

As you have whats referred to as a "many to many" relationship (many events can have many tags) you need an intermediate table that handles assignment - this is called normalisation.
In this case, you need 3 columns: AssignmentID, EventID, TagID
Put all your tags in the same database table, but flag each one as either primary or secondary and handle the 1 primary tag per event within your code outside of the database.
For example your tag table could look like:
ETagID ETagName ETagColour ETagPrimary ETagDel
1 first red 1 0
2 second blue 0 0
3 third green 0 0
4 forth yellow 1 0
5 fifth orange 0 0
6 sixth white 0 0
and your assignment table:
AssignmentID EventID TagID
1 1 1
2 1 2
3 1 5
4 2 4
5 3 4
6 3 1
7 4 4
As your code outside of the sql handles the insertions in the first place, you can now query your tables using joins to pull out the event + tags for that event
SELECT ETagName, ETagColour FROM TagTable
JOIN AssignmentTable on AssignmentTable.TagID = TagTable.ETagID
JOIN EventTable on EventTable.EID = AssignmentTable.EventID
WHERE EventTable.EID = <some value> AND TagTable.ETagDel = 1
This would select all the tag names and colours for that specific event that aren't deleted.
An important thing to note is not to overcomplicate things. If your primary and secondary tags store the same info except for being either primary or secondary, then its pointless separating them into individual tables. Flagging them like I mentioned will be sufficient and reduces the number of tables required.
Hopefully this points you in the right direction moving forwards
Update:
As per the recent comment, you can handle the allocation of the primary tag within the assignment table. Create the same table as above, but include the primary flag column too
AssignmentID EventID TagID PrimaryFlag
1 1 1 1
2 1 2 0
3 1 5 0
4 2 4 1
5 3 4 1
6 3 1 0
7 4 4 1
then within the query, you can also select the status of the tag using a slightly modified version of the one written before:
SELECT ETagName, ETagColour, AssignmentTable.PrimaryFlag FROM TagTable
JOIN AssignmentTable on AssignmentTable.TagID = TagTable.ETagID
JOIN EventTable on EventTable.EID = AssignmentTable.EventID
WHERE EventTable.EID = <some value> AND TagTable.ETagDel = 1
if you want to make sure the primary tag appears at the top of that list, you can also bolt on
ORDER BY AssignmentTable.PrimaryFlag