Inserting Into and Maintaining Many-to-Many Tables

Question

SQLite3 user.

I have read thru numerous books on relational DBs and SQL and not one shows how to maintain the linking tables for many-to-many relationships. I just went through a book that went into the details of SELECT and JOINS with examples, but then glosses over the same when many-to-many relationships are covered. The author just showed some pseudo code for a table, no data, and then a pseudo code query--WTF? I am probably missing something, but it has become quite maddening.

Anyways, say I have a table like [People] with 3 columns: pID (primary), name and age. A table [Groups] with 3 columns: gID (primary), groupname and years. Since people can belong to multiple groups and groups can have multiple people, I set up a linking table called [peoplegroups] with two columns: pID, and gID both of which come from their respective tables.

So ,how do I efficiently get data into the linking table when INSERTING on the others and how do I get data out using the linking table?

Example: I want to INSERT "Jane" into [people] and make her a member of group gID 2, "bowlers" and update the linking table {peoplegroups] at the same time. Later I want to go back and pull out a list of all of the bowlers or all the groups a person is part of.

Answer 1

If you already don't use primary and foreign keys (which you should!) I think you may need to consider using triggers in your design as well? So if you have a specific set of rules (e.g. if you want to create Jane with id = 1 and choose existing group 2, then after insert jane into people automatically create an entry pair personid=1,groupid=2 in the table peoplegroups. You can also create views with specific selects to see the data you want, for example if you want a query where you only show the peoples names and groups names you could create a view 'PeopleView':

SELECT P.PersonName, G.GroupName
FROM People P
INNER JOIN PeopleGroup PG ON P.PersonID = PG.PersonID
INNER JOIN Group G ON G.GroupId = PG.GroupID

then you can query 'PeopleView' saying

SELECT * FROM PeopleView WHERE GroupName = 'bowlers'

Answer 2

When inserting new data into the tables mentioned, the "linking" table that you are referring to needs to contain both primary keys from the other tables as foreign keys. So basically The [People] tables (pID) and the [Groups] table (gID) should both be foreign keys in the [PeopleGroups] table. In order to create a new "link" in [PeopleGroups] the record has to already exist in the [People] table as well as the [Groups] table BEFORE you try and create the link in the [PeopleGroups] table. I hope this helps