Answer
(ProjectID, EmployeeName)
Work Step by Step
To determine the primary key of the table
PROJECT(ProjectID, EmployeeName, EmployeeSalary) we need to see what uniquely identifies each row.
Analyze the table:
ProjectID — identifies a project, but a project can have many employees working on it.
EmployeeName — identifies an employee, but an employee can work on multiple projects.
EmployeeSalary — a property of the employee, same across projects.
Determine candidate key:
Since one employee can work on multiple projects and one project can have multiple employees, neither ProjectID nor EmployeeName alone uniquely identifies a row.
However, the combination of ProjectID and EmployeeName uniquely identifies a row:
(ProjectID, EmployeeName)
This is because each row represents one employee working on one project, and that pair is unique.
So the primary key is
(ProjectID, EmployeeName)
