I have 3 tables:
Employees T
emp_id | name | address
Department T
dep_id | name
Salaries T
emp_id | dep_id | month | year | salary
For each table, what are the primary keys and the foreign keys?
My answer:
Name of the table | PK | FK|
Is my answer correct?
The Employees and Department tables have primary keys (which you got correct), but no foreign keys.
In Salaries, both emp_id and dep_id are foreign keys.
There's no single primary key on Salaries, although emp_id, dep_id, month and year could possibly be a composite key (as the combination of these 4 would always be unique, assuming an employee is only salaried once per month by a department :) ).
Do you have the create script of the tables? With only this information, you can't do anything but guessing by the names of attributes...
Not really. The foreign key and primary keys have nothing to do with names. You will need to use the DESCRIBE syntax as explained here to find what the keys are.
Based on the names I would make these assumptions:
Employees T
emp_id | name | address
PK: emp_id
FK: none
Department T
dep_id | name
PK: dep_id
FK: none
Salaries T
emp_id | dep_id | month | year | salary
PK: none
FK: emp_id, dep_id
It looks like two tables (Employees and Department) and linking (junction) Salaries table for implementing many-to-many relationship.
As stated in other answers, you need only primary keys for both Employees (PK emp_id) and Department (PK dep_id) tables, no foreign keys.
Table Salaries mast has foreign keys on emp_id, dep_id columns. For primary key you have two choices. Primary Key enforces uniqueness of the column on which they are defined. So, or no primary key at all, or compound primary key (emp_id, dep_id, month, year); the order of columns depends on you needs (select queries).