SQL Queries for a database - sql

Question

I have an assignment on DB. I am having some problems with my queries. I am new to SQL.
Here's my tables:
CREATE TABLE Employee (
employee_name varchar(20) NOT NULL PRIMARY KEY,
street varchar(30) NOT NULL,
city varchar(20) NOT NULL
)
CREATE TABLE Works
(
employee_name varchar(20) NOT NULL,
department_name varchar(20) NOT NULL,
title varchar(10) NOT NULL,
salary int NOT NULL,
PRIMARY KEY (employee_name),
FOREIGN KEY (employee_name) REFERENCES Employee,
FOREIGN KEY (department_name) REFERENCES Department
)
CREATE TABLE Department
(
department_name varchar(20),
cityvarchar(20),
PRIMARY KEY (department_name)
)
CREATE table Manages(
employee_name varchar(20) not null,
manager_name varchar (20) not null,
PRIMARY KEY(employee_name),
FOREIGN KEY(employee_name) REFERENCES Employee
)
Find employee names who live in different cities from their department.
Here's my code:
SELECT E.employee_name
FROM Employee E
JOIN Department D
ON E.city = D.city
WHERE E.city <> 'Roma';
Find employee names who live in the same city and in the same street with their managers.
Here's my code:
SELECT E.employee_name
FROM Employee E
INNER JOIN Department D
ON E.city = D.city
INNER JOIN Manages M
ON E.employee_name = M.employee_name
WHERE city='Roma' AND street='Main street';
What am I doing wrong?
Thank you in advance! :)

Answer 1

Here's how you might accomplish the first task (employees who live in a city other than that of their department):
SELECT E.employee_name
FROM Employee E JOIN Works W
ON E.employee_name = W.employee_name
JOIN Department D
ON W.departent_name = D.department_name
WHERE D.city <> E.city;
In order to determine the department in which a given employee works, you need to join to the Works table.
I have to admit that the vertical partition of employee data into Employee and Works tables is a bit odd - there is no real reason for these two to be separate tables (it's a 1-1 relationship). For that matter Manages need not be a separate table either.
Hopefully this can get you on the right path for the second query.