February 5, 2014 at 5:33 am
hi everybody
I have two table that have many to many relationship so i add table between them.and this table column is Id of each table.my question is i want this table info be unique(not duplicated) and it is better this two id in this table be unique or be primary key with same time that are foreign key????
February 5, 2014 at 8:19 am
vahid.arr (2/5/2014)
hi everybodyI have two table that have many to many relationship so i add table between them.and this table column is Id of each table.my question is i want this table info be unique(not duplicated) and it is better this two id in this table be unique or be primary key with same time that are foreign key????
I am missing something in translation but if I understand you correctly you have a bridge table to prevent a many to many relationship?
An example might be Employees and Departments where a given employee can belong to more than 1 department. You could create a EmployeeDepartment table with two columns (EmployeeID and DepartmentID). Each of those columns have a foreign key reference. Additionally the primary key for EmployeeDepartment would be a composite key.
Is that the question, and if so does that make sense?
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February 5, 2014 at 8:37 am
Assuming I understand the question, yes, that's not only possible, it's the right way to get the job done.
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February 6, 2014 at 5:49 am
my question is as you say in example.it is better EmployeeID and DepartmentID be both primary key as they are foreign key or i unique them to prevent duplicate info???
February 6, 2014 at 6:10 am
In the sample the EmployeeID is the primary key in the Employee table. The DepartmentID is the primary key in the Department table. The primary key in the EmployeeDepartment table (the bridge table) is a composite of both the EmployeeID en the DepartmentID column.
Because this bridge table has a one-to-many relationship to both the Employee and the Department table, you have in effect created a many-to-many relation between these two tables. The records in the EmployeeDepartment table (the bridge table) are unique.
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