Question

Which is good practice? To store data as a comma separated list in the database or have multiple rows?

I have a table for accounts, classes, and enrolments.
If the enrolment table has 3 fields: ID, AccountID and ClassID, is it better for ClassID to be a varchar containing a comma separated list such as this: “24,21,182,12” or for it to be just an int and have one entry per enrolment?

Answer 1

tldr: Don’t do this. That is, don’t use a “packed array” here.

Use a correctly normalized design with “multiple rows”. This is likely a good candidate for a Many-to-Many relationship. Consider this structure:

Classes 1:M Enrollments(Class,Student) M:1 Students

Following a properly normalized design will reduce pain. In addition, here are some other advantages:

• Referential integrity (use InnoDB)
  • Consistent model described with relationships
  • Type enforcement (can’t have "foo,,")
• JOIN and query without needing custom code
  • “What are the names of the students in class A?”
  • “Who is taking more than one class?”
  • Columns can be useful indexed (query performance)
  • Generally faster than handling locally in code
• More flexible and consistent
  • Can attach attributes to enrollments such as status
  • No need to have code to handle serialization at access sites
  • More accommodating of placeholders and ORMs