The created database serves a company in the telecom industry. It enables the management of branches, clients, employees, their positions, salaries and its taxations, managers of particular branches, stocks (phones that are to be sold in a particular branch), information about the phone producers and their country. The entities and relationships between them are the following:

1. Phones - Stocks - Branch, N:N relation. The primary key in the Phones table represents its ID. Moreover, the table includes the names of particular phone models. The primary key in the Branch table is the ID of the respective Branch. In both tables the primary keys enable the diversification of each item (phone model and branch), which is unique. As these tables are related to the Stocks table where pieces of particular models in distinct branches are shown, model_of_phone and branch_of_stock are the foreign key in the Stock table - each model_of_the phone and branch_of _stock can be repeated in the table more than once.

2. Branch – Employees – Position, N:N relation. The table Employees has two foreign keys: the branch of each employee (branch_emp) and the position of the employees (position_of_empl), which connect this table with two other tables: Branch and Position. ID is a primary key in both the Branch table and Position table as a unique value. This relationship enables to connect employees with the branch they work in and their position. There can be multiple employees on the same position within the same branch.

3. Managers - Branch, 1:1 relation. Each manager is responsible for one branch. For that reason, branch_ of_manager is a primary and foreign key at the same time, which is connected with the Branch table through the ID of the branch.

4. Salary – Position, 1:1 relation. Depending on the position in the company the salary varies. Therefore, each position is connected with one amount. The ID in the Salary table is a primary key and the salary_for_the_position in the position table is a foreign key.

5. Clients – Branch, N:1 relation. Each Client is assigned to a particular branch (e.g. they signed the contract in this branch). The ID of the Branch table (primary key) is connected with branch_of_the client column, which serves as a foreign key as many clients are assigned to the same branch.

6. Size_of_branch – Branch, 1:N relation. The area of each branch can be defined as either very small, small, medium, big or very big. Each size has its own ID, which is the primary key, but the area in the branch table represents its foreign key due to the fact that different branches can be within the same area.

7. Taxation_of_salary – Salary, 1:N relation. For different amounts (salaries) the same effective rate of tax can be implemented. The ID of taxation_of_salary is unique. Hence it serves as a primary key. Tax in the salary table is a foreign key.

8. Country - Producers, 1:N relation. This relation enables to connect Country names in the Country table (ID is a primary key because each value is unique) with phone producers. Few producers can have the same country of origin. Consequently, country_of_producer is a foreign key in the Producers table.

9. Producers – Phones, 1:N relation. The relation shows the producers of particular phones (e.g. the iPhone 7 is produced by Apple). Each producer in the Producers table is different, which is why ID represents a primary key. However, different phones can have the same producers so supplier in the Phones table is a foreign key.