Cardinality and SQL requests

Question

1.00/5 (1 vote)

See more:

Hello,

I have a Teacher table and a Pupils table.
A teacher can have 5 to 30 students and a student can have only one teacher. With SQLite, how can I check this cardinality with an SQL query?

For example, how do you prevent a user from adding more than 30 students to the Teacher table or from assigning multiple teachers to a student by using C# and SQLite?

Thank you for your help.

What I have tried:

I tried with some SQL requests (SQLite), but it seems there is not a check of cardinality. What is the way to check cardinality in SQL ?

Posted 4-Jul-21 23:10pm

Member 14817640

Updated 5-Jul-21 0:43am

Add a Solution

Comments

FranzBe 5-Jul-21 6:15am

This link might be helpful: https://stackoverflow.com/questions/56876547/sql-query-find-records-with-field-that-has-cardinality-greater-than-one

1 solution

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

CHill60 · Answer 1 · 2021-07-05T00:43:00

All you need is the COUNT function see SQLite COUNT Function: Count Items In A Group[^]
Before linking a student to a teacher, query to see if the student already has a teacher - you will also need to know how to use SQLite WHERE - Filter Rows in a Result Set[^].
If teacher is already assigned, then do not add student to teacher otherwise query to see how many students the teacher already has.
If teacher already has 30 then do not add student to this teacher.
How you then handle the situation is up to you.
If you are still stuck then come back with the code you are trying to use and we will try to help further, but without seeing your code it is difficult to be more specific (I will not just give you the code as this looks like homework)