Relationship between entities in database

Question

When describing relationships between entities have I understood it correct if it the occurrence of a primary key in the table, lets say A has in table B that determines whether it is a one-to-one, one-to-many and so on?

If table B has an attribute with foreign keys, and each row in table B has a reference to the same table in row A, then this is a one-to-many relationship, correct?

Thanks in advance

Answer 1

You're mixing up tables and rows in your question, so it'm trying to explain a little more.

If Table B has a column for foreign keys, and each of the rows in this column has a foreign key that refers to the primary key column of a row in Table A, then this is a one-to-one relationship.

However (and this is what you're stating), if several foreign keys in rows of Table B refer to one row in another Table A, then this is a one-to-many relationship, as there are multiple B's referring to one A.

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Example

Employees are assigned a type of computer.

Table Employees:

ID    Name  Computer
==============
1     Mike   1
2     Tom    3
3     Joe    1
4     Emily  4

Table Computer Types:

ID    Name
==============
1     HP
2     Macbook Pro
3     Dell
4     Alienware

Then this is a one-to-many relationship telling us that one computer type can be assigned to many employees, i.e. Joe and Mike are using HP computers.