[ACCEPTED]-Can a junction table (join table) also be used for a one-to-many relationship?-junction-table
There is not any reason why a junction table 4 couldn't be used for a one-to-many relationship. The 3 question is usually one of performance. Why 2 make the database join an additional table 1 when it is unnecessary?
Yes, it is still possible to store and enforce 16 one-to-many relationship in a junction table.
In 15 your example you are not enforcing any constraints 14 on the
UserOrders junction table, so a single order 13 can belong to two users (assuming that's 12 incorrect). To enforce that you could make 11
OrderKey be the primary key of the
UserOrders junction table 10 (or have a unique constraint on that column). Technically 9 that will just become a many-to-one relationship between 8
Users, while having one-to-one relationship between 7
I can only think about one reason 6 for designing the many-to-one relationship using junction 5 table - if you plan to allow the many-to-many relationship 4 in future and don't want to deal with data 3 migration. But in the mean time you will 2 pay the cost of storing and joining with 1 additional table.
This would be many-to-many:
CREATE TABLE UserOrders (UserLogin varchar(50) REFERENCES Users (UserLogin), OrderKey varchar(50) REFERENCES Orders (OrderKey), PRIMARY KEY (UserLogin, OrderKey));
This would be 2 one-to-many (one user has many orders):
CREATE TABLE UserOrders (UserLogin varchar(50) REFERENCES Users (UserLogin), OrderKey varchar(50) REFERENCES Orders (OrderKey), PRIMARY KEY (OrderKey));
Note 1 the difference in the PRIMARY KEY constraint.
Once you've built a table, it really doesn't 17 have a type of "Junction" table, "associative" table, "join" table 16 -- it's just a table.
We use these terms 15 to describe a specific reason why an entity 14 (and resulting table) was initially created. Associative 13 entities are created, initially, to resolve 12 a many-to-many situation. But these tables 11 quite often have attributes of their own 10 (such as the time of the association, a 9 reason for the association, etc.). So SQL 8 Server, Oracle or your code has no reason 7 to know why a table was created...just that 6 it's a table.
From a technical point of view, there 5 really isn't any difference between an associative 4 table and any other table.
So these tables 3 can fill any role that any other table can 2 fulfill. There are no rules around how 1 other tables can also be related to them.
You can enforce de "one" constraint in thee 8 join/junction table adding a unique constraint 7 (or making it the primary key of the join 6 table, because just that atribute itself 5 identifies the relationship) to the column 4 that is a foreign key to the "many" side. That 3 is because you want rwos in the many side 2 have only one relationship and relationships 1 are stated in the join/junction table.
I think you got the concept wrong - Here 1 is the simple explanation if it could help: To achieve a Many-Many relationship between two tables(say, A and B), we need to take the help of a junction table(say, table c) which will have one-many relationship with both tables A and B.
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