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Editorial Team
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Asked: 2026-05-25T21:42:41+00:00

I’m constructing a schema for a MySQL database. I have a table called ‘entry’

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I’m constructing a schema for a MySQL database. I have a table called ‘entry’ (the supertype). An entry can be either a ‘photo’, ‘essay’, or ‘video’ (subtype). Each subtype has different properties/columns.

My current design calls for an entries table, and a separate table for each of the three sub-types.

Schema

The sub types are associated with the a record in ‘entries’ via a foreign-key to the entries table’s id attribut. My question is, how can I modify this design to restrict an entry to being associated with only one type of subtype. Currently, multiple subtypes can be associated with the same entries record.

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1 Answer

  1. Editorial Team

    You cannot easily do this declaratively in SQL. What you want to do is put a CONSTRAINT on the primary key that’s shared across all four tables that the key must exist in entries (that’s OK, it’s a PRIMARY KEY) and that it must not exist in either of the other two tables. That second part has no corresponding constraint type in SQL.

    You’re basically stuck with TRIGGERs. (You can do this in some other engines with a CHECK CONSTRAINT that contains a sub-query, but I don’t think MySQL supports CHECK CONSTRAINTs at all and some engines will not look outside the current row when evaluating a CHECK CONSTRAINT).

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