Member Avatar for azapovjednik

Hello all,

I need to make some database... and lets say there is a student table, and the student table has many columns, a lot of columns.... they are all unique to the primary key, so dont worry about the normal forms...

what I am in a doubt is: there is some data, that could be put in another table, but it should be a one-to-one relationship!
because, one student can have only one dossier (for instance: sign_up_date, student_status, etc)

so, my question is:
- should all the data be put into one table called students?

  • or should I separate the data into two tables students and student_dossier? In this case, student_dossier *table should have one primary key named student_id, just like in *students table

I am a bit confused, so I hope you could help me to understand it... Thank you very much in advance

  • 3 Contributors
  • 7 Replies
  • 260 Views
  • 1 Day Discussion Span
  • Latest Post Latest Post by azapovjednik

Dani AI

Generated

Short answer: decide by how the data is used, not by the abstract “one‑to‑one” label. was right to say “it depends,” and pointed to the practical criterion: access patterns. Splitting into a separate dossier table is a tool — useful in specific cases, unnecessary in others.

Typical reasons to split (examples that go beyond simple column count):

  • Large or infrequently read columns (BLOB / long TEXT) that bloat the common row and reduce cache efficiency.
  • Different retention/backup or security requirements for the dossier data.
  • Many optional fields that would otherwise be NULL for most rows (sparse attributes).
  • Subtype data: a subset of students needs extra fields that most do not.
  • Different update frequency: isolating frequently updated columns can reduce contention.

Practical MySQL notes and two common implementations:

  • The simplest way to enforce 1:1 is to make the child table use the parent key as its PRIMARY KEY (no extra surrogate key). That keeps indexes small and makes joins efficient.
  • An alternate approach is a child table with its own PK and a UNIQUE FOREIGN KEY to the parent; this is more flexible if the child needs its own identity.

Example patterns:

-- child uses parent PK (compact, strict 1:1)
CREATE TABLE student_profile (
  student_id INT PRIMARY KEY,
  sign_up_date DATE,
  status TINYINT,
  FOREIGN KEY (student_id) REFERENCES student(id) ON DELETE CASCADE
);

-- child with surrogate PK + unique FK (more flexible)
CREATE TABLE student_meta (
  id INT AUTO_INCREMENT PRIMARY KEY,
  student_id INT UNIQUE,
  notes TEXT,
  FOREIGN KEY (student_id) REFERENCES student(id)
);

Caveats and rules of thumb: joins on PKs are cheap in InnoDB, but if almost every query needs both halves then the extra JOINs add overhead and complexity. MySQL does not support deferrable foreign keys — the parent row must exist before inserting the child (use a transaction to keep changes atomic). For ’s scenario, split only if dossier fields are logically separate (rare, large, sensitive or subtype); otherwise a single table keeps queries simpler.

Recommended Answers

All 7 Replies

Member Avatar for Reverend Jim

You have to strike a balance between normalization and usability. For a student table, you should keep basic information like

Name
Address
Telephone
StudentID (which will be used as a foreign key in other tables)
Status

In one table. This is information that is used to uniquely identify a student. Academic information such as courses, grades, etc. belong in a separate table. These other tables will refer back to the student table via the StndentID.

Member Avatar for azapovjednik

I am aware of the need to put separate data into separate table.... like grades, etc...
but what about one to one relationship? in what instance do we use it? is it like having all the data in one table...

I am looking in Access 2007, in order to create one to one relationship, two tables need to have one and only and the same primary key, like StudentID

so I wonder, does separating data in that way only help to alleviate the burden of one table? Is that it? Is that the purpose of having two tables with the same ID field?

Is the database gonna be slow that way?

I hope you can help me understand that concept better, thanks again.

Member Avatar for Reverend Jim

Tables do not need to be restricted to a one to one relationship. Databases would not be very useful if this was the case. For example, one student can take many courses. In that case you might have (as an example)

StudentsTable
-------------
StudentID (PK)
LastName
FirstName
.
.
.

CourseTable
-----------
CourseID (PK)
CourseName
.
.
.

StudentCourseTable
------------------
StudentID (FK)
CourseID (FK)
Grade
.
.
.

Because you need to have a primary key to uniquely ID a record in the StudentCourse table, the primary key (PK) is a compound key consisting of the StudentID and CourseID which are also foreign keys (PK) because they link back to other tables. In real life it gets much more compliciated because you would need at least two more tables for courses. The primary would detail each course but you would need another one-many because each course would be offered in multiple slots and multiple terms, and yet another to specify the prerequisites.

Edited by Reverend Jim
Member Avatar for azapovjednik

is my English that bad? All I was asking was: one to one relationships as opposed to all the data in one table, instead of two tables.

Member Avatar for nauticalmac

I think it would depend on how you would access the data under normal use. Would all your queries need to access all the data? Or might there be a subset that is accessed more frequently where having it in a separate table may marginally improve performance?
I think splitting the data into two tables is something I would only consider if the number of records and the number of fields was very large.

Member Avatar for Reverend Jim

is my English that bad? All I was asking was: one to one relationships as opposed to all the data in one table, instead of two tables.

I suppose I could have just answered "it depends on the data" but that really wouldn't have been useful.

Member Avatar for azapovjednik

I see, thanks nauticalmac.... that makes sense then..... since there are many fields, and also this data in a separate table would be considered as one group... then it makes sense. Thank you.

Be a part of the DaniWeb community

We're a friendly, industry-focused community of developers, IT pros, digital marketers, and technology enthusiasts meeting, networking, learning, and sharing knowledge.