My teacher asked us to select from a 1:1 table called Employee; the supervisor and their role, and each employee they supervise with their role (employee is the primary key and those are the only 3 values in the table).
A 1:1 relationship are two tables with only one possible matching id.
It assumes that each supervisors only has one employee. so your query would look something like this.
SELECT Employee.Name, Employee.Role, supervise.Name, supervise.Role
FROM Employee
INNER JOIN supervise
ON Employee.EmployeeId = Supervise.EmployeeId
However, do note that in a real world context. This should have been a 1:N (One-To-Many) as each supervisors can have many employees.
Usually a 1:1 relationships are only used when you want to extended a table that you have no access or cannot modify. Otherwise, you would just add more columns to that original table. (or if you are working with a very old database system and you reach the max number of columns)
Related
Let's assume I am building the backend of a university management software.
I have a users table with the following columns:
id
name
birthday
last_english_grade
last_it_grade
profs table columns:
id
name
birthday
I'd like to have a third table with which I can determine all professors teaching a student.
So I'd like to assign multiple teachers to each student.
Those Professors may change any time.
New students may be added any time too.
What's the best way to achieve this?
The canonical way to do this would be to introduce a third junction table, which exists mainly to relate users to professors:
users_profs (
user_id,
prof_id,
PRIMARY KEY (user_id, prof_id)
)
The primary key of this junction table is the combination of a user and professor ID. Note that this table is fairly lean, and avoids the problem of repeating metadata for a given user or professor. Rather, user/professor information remains in your two original tables, and does not get repeated.
Table Department:
DeptId
Name
Table Team
TeamId
*DeptId (FK)
Name
Table Employee
EmpId
*DeptId
*TeamId
I am making some updates on an old project, but I don't know why the old programmer designed those tables like this (like putting both DeptId and TeamId in the Employee table).
I find this useless because I can get the department from the Team table, and there's no need to put both FK IDs into the Employee table, TeamId is enough.
Is there any other reason that could force me to put both FKs in that table?
Thank you.
As the data model is written, an employee could be a member of a team that is not in his or her department.
That is probably possible, for instance, if the employee is temporarily on loan.
My bigger problem with the data model is that the relationships between employee and team and employee and department vary over time. So, I would have three tables for each entity. The only relationship in the tables would be between team and department (because that presumably does not change over time).
Then I would have two junction tables, one employeeDepartments and one employeeTeams that capture the changing relationships over time.
I looked for this and did not find a solution that would apply to my scenario.
I'm building a database of game devs and I wish to generate a connections table:
I have the following:
Employee
(
name, date of birth, department they work at, task they do
)
Department
(
department name
)
Task
(
task name
)
and I need to generate a connections table that shows which department contributes to which task. I would do that by checking for each employee their department (only one) and task (also only one) and upon a match, the department contributes to that task.
That is the idea but I have to clue how to code it using Oracle
SELECT DISTINCT "department they work at", "task they do"
FROM Employee;
You should first work out an entity-relationship diagram, that lists the entities you use and with what attributes (and which primary keys), and the relations between those entities. Relationships can be: 1-to-1, 1-to-many and many-to-1, and many-to-many.
In the last case (M:N relation), the implementation in database tables requires an extra table to record such a M:N relationship.
The way to implement a 1:N relationship in a table is adding a foreign key in the child table to the primary key of the parent table.
EDIT: I see that you now supplied some details, and it is clear now that EMPLOYEE table is in fact the connection table, so you could simply query that table and show the DEPTID and TASKID (both the primary keys of their respective tables) to have a connection between departments and tasks. See the query in the other answer, and just add an ORDERBY on DEPTID, to show results in the order of DEPTID.
I'm not sure about the best way to express a relationship between models in my Laravel 4 application. I have three models that I am using to store information in a database: Employer, Employee and User. Basically, the Employer and Employee models are just going to contain meta information about the User model. I think the following expression is the best I can think of:
Employer has many Employee
Employee belongs to one Employer
Employer belongs to one User
Employee belongs to one User
In the database, I am going to have a user_id foreign key in both the employers and employees tables that reference the id field in the users table. Is this the best way to model this domain?
It depends. Can an employee have more than one employer? Can an employee be employed at more than one place? Do you want to delete records if an employee leaves (and lose history)?
If you can delete the employee record when the employee leaves you can just have an employer_id field in the employee table. But just be prepared that you won't be able to easily add capabilities if you need them in the future.
If you want to prepare for the future: You might want to have a table that says
employer_id
employee_id
start_date
end_date
And even then if an employee comes and then leaves you have records with the same employer_id, employee_id. So that makes it look like you need an internal key and have say:
employment_record_id // this is the primary key
employer_id
employee_id
start_date
end_date
I'm not sure exactly what users are but ask yourself the same types of questions:
Can a user exist with an employee record
Can you have employees without a user record
Can a employees user record change
Can a users employee record change
Tables:
Students
Professors
Entries (there’s no physical table intry in the database for entries yet, this table is on the front-end, so it is probably composed from multiple helper tables if we need them. Just need to create valid erd)
Preambula:
One student can have an association to many professors
One professor can have an association to many students
One entry can have 0,1 or more Students or professors in it.
Professor is required to be associated with one or more students
Student is not required to have an association with any professor
It should be more like this (front-end entry table):
Any professor in this table must have an associated name in the table.( For example Wandy is associated to Alex)
It is not required for student (but possible) to have associated professors in this table
One row (for example Linda (Student), Kelly (Professor),Victor (Professor))
Cannot be associated between each other in any manner.
But it is absolutely fine if Linda associated with David.
The problem is that I do not quite understand how one column can have ids of different tables (And those are multiple!) And do not quite understand how to build valid erd for that.
I will answer any additional questions you need. Thanks a lot!
If you simply want an association between Students and Professors - just make a many-to-many relationship in ERD. In logical (relational) schema it will make an intermediate table with foreign keys to Student and Professor tables.
But from your example it looks like you need to design the DB for your "PeopleEntries", which is not straightforward. ERD seems to have the following entities:
Students(ID, name)
Professors(ID,
name)
PeopleEntries(ID, LoveCats,
LoveDogs, LoveAnts)
Relationships (considering people cannot appear in entries more than once):
Students Many - 1 PeopleEntries
Professors Many - 1 PeopleEntries
Students Many - Many Professors
Relational schema would contain tables (foreign keys according to erd relationships):
Students(ID, name, PeopleEntryID FK)
Professors(ID, name, PeopleEntryID
FK)
PeopleEntries(ID, LoveCats, LoveDogs,
LoveAnts)
StudentProfessor(StudentID FK,
ProfessorID FK)
I don't know how to implement the constraint, disallowing association between people from the same entry, on conceptual level (ER-diagram). On physical level you can implement the logic in triggers or update procedures to check this.
As per my quick understanding,
Create a table with following columns
PersonName
Designation
.....
Create one more table
PersonName
LinksTo
In the second table each person entry will have multiple records based on the relation
You want a junction table:
ID StudentID ProfessorID
0 23 34
1 22 34
2 12 33
3 12 34
In the table above, one professor has 3 students, one student has two professors.
StudentID and ProfessorID should together be a unique index to avoid duplicate relationships.