MySQL inner join with left outer join - mysql

Question

I have 4 tables as below.
1. emp
- emp_id
- name
- contact_id
2. contact
- contact_id
- name
3. emp_projects
- prj_id
- emp_id
4. project
- prj_id
- name
Relationships :-
emp to projects - One to Many
I need a query joining all these tables.
Conditions
contact_id column in emp table can be empty so get contact details only if the column is not empty
Employee can be part of Zero or many projects.
Query needs to return the project name as well.
Query that I currently have,
select e.,ep. from emp e left outer join emp_projects ep on e.emp_id = ep.emp_id where e.emp_id=1
Need to join contacts and projects table as well here.
Sample data,
1. emp
| 1 | Mike | 1 |
| 2 | John | - |
2. contact
| 1 | Alex |
3. emp_projects
| 1 | 1 |
| 2 | 1 |
4. projects
| 1 | Test |
| 2 | Mail |
Expected output :-
For emp id 1,
1(emp_id), Mike(emp_name), 1(contact_id), Alex(contact_name), 1(proj_id), Test(Prj_Name)
1(emp_id), Mike(emp_name), 1(contact_id), Alex(contact_name), 2(proj_id), Email(Prj_Name)
For emp id 2, ( Contact column is empty for emp 2 and emp 2 is not associated with any project )
1(emp_id), John(emp_name), -(contact_id), -(contact_name), -(proj_id), -(Prj_Name)

Answer 1

Query and DDL script is added below. 1 recommendation, use the primary key in the mapping table or have a composite key ( emp_id, project_id )
select
employee.id as employee_id,
employee.name as employee_name,
contact.id as contact_id,
contact.name as contact_name,
project.id as project_id,
project.name as project_name
FROM employee employee
LEFT OUTER JOIN contact contact on contact.id = employee.contact_id
LEFT OUTER JOIN employee_projects employee_project on employee_project.employee_id = employee.id
LEFT OUTER JOIN project project on project.id = employee_project.project_id
CREATE TABLE IF NOT EXISTS `employee`
( `id` int(6) unsigned NOT NULL
, `contact_id` int(6) unsigned NULL
, `name` varchar(200) NOT NULL
, PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;
INSERT INTO `employee` (`id`, `contact_id`, `name`) VALUES
('1', '1', 'Mike'),
('2', null, 'John');
CREATE TABLE IF NOT EXISTS `contact`
( `id` int(6) unsigned NOT NULL
, `name` varchar(200) NOT NULL
, PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;
INSERT INTO `contact`
(`id`, `name`) VALUES
('1', 'Alex');
CREATE TABLE IF NOT EXISTS `project`
( `id` int(6) unsigned NOT NULL
, `name` varchar(200) NOT NULL
, PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;
INSERT INTO `project` (`id`, `name`) VALUES
('1','Test'),
('2', 'Mail');
CREATE TABLE IF NOT EXISTS `employee_projects`
( `id` int(6) unsigned NOT NULL
, `employee_id` int(6) unsigned NOT NULL
, `project_id` int(6) unsigned NOT NULL
, PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;
INSERT INTO `employee_projects`
(`id`, `project_id`, `employee_id`) VALUES
('1', '1', '1'),
('2', '2', '1');

Answer 2

I think the below query will to the trick:
select e.,ep.,c.,p.
from emp e
left outer join emp_projects ep on e.emp_id = ep.emp_id
left outer join contact c on c.emp_id = e.emp_id
left outer join project p on p.proj_id = ep.proj_id
where e.emp_id=1