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Editorial Team
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Asked: 2026-05-18T01:13:37+00:00

I have a problem where, due to previous people’s poor design decisions, I’m forced

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I have a problem where, due to previous people’s poor design decisions, I’m forced to have two columns in a linking table that should always be mutually exclusive. I’ve seen SQL-Server: Define columns as mutually exclusive but MySQL doesn’t enforce CHECK constraints so I’m looking for other options. Is there any way to enforce mutual exclusion between two columns?

Let me explain in case someone sees a better solution.

Previous dev/admins have constructed two tables that record basically the same type of things. I have a new a table that is 1 -> many in theory to the thing they both represent. So my two options seem to be to create either two bridging tables or 1 bridging tables with two external key references. The two tables seems like an ugly solution to me that I was hoping to avoid but unenforced mutually exclusive fields leaves me open to easy corruption.

Concrete example since I can’t share my real case:

Table 'full_time_students' stores full time students
Table 'part_time_students' store part time students

Table 'class' will store class info
Table 'class_students' should store which students are in which class

In an ideal world, I would combine the two student tables but there’s a lot of code that would break and I don’t have the time/will to go fix all of that so I’m forced for now to deal with the existent schemas/data. I’m using InnoDB so I was already planning on having foreign key constraints on the student_ids but like I said, if I do one table with two columns I open myself up to the corruption of having one row with both ids.

Just writing this out almost convinces me to reconsider the 2 table solution…

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1 Answer

  1. Editorial Team

    Here’s a thought to fix the problem without breaking much code. Create a newtable students and populate with the data from both the other tables and a flag field to indicate full or part time. Then create views that have the names of the full time students and part time students table. This means all selects will still work perfectly and all you will have to do is fix the insert and update code to go to the new table.

    Or you could enforce your rules in a trigger.

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