Relational Database (H2, Java): How do I constrain a foreign key to NOT match another foreign key in the same table?

Question

Simple question. Just wondering if this can be done without me having to enforce this constraint manually in my Java code. These two foreign keys (together in the same table) both refer out to another table, but for each row, they must not be allowed to point to the same foreign item.

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Answer 1

You can use a check contraint to enforce that two columns have different values:

ALTER TABLE TableName 
    ADD CONSTRAINT ConstraintName
    CHECK fk1 <> fk2