How can I select two columns from two tables? - sql

Question

These are my tables
create table employees
(emp_no integer not null,
emp_name char(25),
age integer,
department_id integer not null,
salary integer,
CONSTRAINT employees_pk PRIMARY KEY(emp_no),
CONSTRAINT fk_Department FOREIGN KEY(department_id)
REFERENCES Department(department_id));
insert into employees values ( 15,'sara',30 ,101 ,2000 )
insert into employees values ( 12,'maha',29 ,104 ,3000 )
insert into employees values ( 14,'ahmad',24 , 102,4400 )
insert into employees values ( 11,'ali', 27, 103, 2500)
insert into employees values ( 13,'nora', 35, 101,3500 )
create table Works
(emp_no integer not null,
hurs integer,
department_id integer not null,
CONSTRAINT Works_pk PRIMARY KEY(emp_no),
CONSTRAINT Department_fk FOREIGN KEY(department_id)
REFERENCES Department(department_id));
insert into Works values ( 11,7,103)
insert into Works values (12,9,104)
insert into Works values (13,5,101)
insert into Works values (14,10,102)
insert into Works values (15,8,101)
create table Department
(Department_id integer not null,
dep_name char(50),
CONSTRAINT Department_pk PRIMARY KEY(Department_id));
insert into Department values (101,'computer')
insert into Department values (102,'history')
insert into Department values (103,'english')
insert into Department values (104,'physics')
And I want to select the employee names and their department names and these are my commands non worked properly:
select emp_name
from employees
select dep_name
from Department
select emp_name
from employees
union
select dep_name
from Department
select emp_name, dep_name
from Department
cross join employees
SELECT employees.emp_name, Department.dep_name
FROM employees, Department
where employees.emp_name = Department.dep_name
select emp_name, Department.dep_name
from employees
inner JOIN Department on employees.emp_name = Department.dep_name
select
count(distinct employees.emp_name)
from
employees
where
employees.emp_name not in (select distinct Department.dep_name
from Department)
select (employees.emp_name)
from employees
where employees.emp_name not in (select Department.dep_name
from Department)
Also I want to select employee name and sum of their salary who are working hour is greater than or equal 20 and my command didn't work:
select
emp_name, sum(salary)
from
employees
union
select
hurs
from
works
where
hurs >= 20
Where did I go wrong here?

Answer 1

First, to get the workers' name and their department, you should be linking table employees and department using department_id, not name..
select emp_name,Department.dep_name
from employees inner JOIN Department
on employees.department_id = Department.Department_id
and for the employees whose working hours are >=20:
select employees.emp_name
from employees join works
on employees.emp_no=works.emp_no AND works.hurs>=20
and their total salary:
select sum(employees.salary)
from employees join works
on employees.emp_no=works.emp_no AND works.hurs>=20
note: inner join and join are the same thing..
In short, you should realize that linking the tables using join is relatively intuitive..
you are linking them based on the "same" column that the both tables own..
[one of them is a foreign key of the other (primary) key]
UPDATE:
you can display the employees' names and their total salary together as the following, but it would seem redundant..
select employees.emp_name, sum(employees.salary)
from employees join works
on employees.emp_no=works.emp_no AND works.hurs>=20
group by employees.emp_name

Answer 2

To union two SELECT results they have to have
same selected column count
same type of
What you actually want to use is INNER JOIN :
SELECT emp_name,sum(salary) FROM employees
INNER JOIN works ON employees.emp_no = works.emp_no
WHERE works.hurs >= 20

Answer 3

The correct syntax is as follows:
SELECT employees.emp_name,
Department.dep_name
FROM employees, Department
WHERE employees.department_id = Department.department_id
or with shorthand:
SELECT e.emp_name,
d.dep_name
FROM employees as e, Department as d
WHERE e.department_id = d.department_id
A description of what is going on:
The SELECT section should contain a list of columns that you want separated by commas. You can explicitly reference a column from table as shown in the examples.
The FROM section should contain a list of tables you wish to join separated by commas. You can provide shorthand names for the tables, as shown in the second example.
The WHERE section allows you to mark which columns are related between the two tables.