Notes

Question 28: Show all of the project names and the total hours per week worked on each project
SELECT project_name, SUM(hours_per_week)
FROM project
JOIN project_assignment
ON project.project_id = project_assignment.project_number
GROUP BY project_name

Sometimes, the database can have not have the same name for the keys

SELECT p.project_name, SUM(pa.hours_per_week)
FROM project AS p
JOIN project_assignment AS pa
ON p.project_id = pa.project_number
GROUP BY p.project_name

SELECT project_name, SUM(hours_per_week)
FROM project, project_assignment
WHERE project.project_id = project_assignment.project_number
GROUP BY project_name

(the JOIN condition is specified in the WHERE clause. 2 tables in the FROM clause matches them all up

Question 29: Show the name of each project and the name of the department that manages the project

SELECT p.project_name, d.department_name
FROM project AS p
JOIN department AS d
ON p.departmentid = d.departmentid

SELECT p.project_name, d.department_name
FROM project AS p
JOIN department AS d
USING(departmentid)

Using assumes that the two tables to be joined have a column that is the exact same name

Question 30: Show the names of all employees, the projects they work on and the number of hours per week they work on each project.

SELECT e.first_name, e.last_name, p.project_name, pa.hours_per_week
FROM employee AS e
JOIN project_assignment AS pa
ON e.employeeid = pa.employeeid
JOIN project AS p
ON p.project_id = pa.project_number

Question 31: Show the names of all employees, the projects they work on and the number of hours per week they work on each project, and the name of the department that manages the project

SELECT e.first_name, e.last_name, p.project_name, pa.hours_per_week, d.department_name
FROM employee AS e
JOIN project_assignment AS pa
ON e.employeeid = pa.employeeid
JOIN project AS p
ON p.project_id = pa.project_number
JOIN department AS d
ON e.departmentid = d.departmentid

Question 32: Show the names of all of the departments and the employees that work in each department

SELECT d.department_name, e.first_name, e.last_name
FROM department AS d
JOIN employee AS e
ON d.departmentid = e.departmentid

However, this query does not show the Shipping Department

Use a LEFT JOIN (show all of the records to the table to the left, department)
Show all of the records in the department table

SELECT d.department_name, e.first_name, e.last_name
FROM department AS d
LEFT JOIN employee AS e
ON d.departmentid = e.departmentid

Question 33: Which departments have no employees?