modeling a "one-to-one" relation in a database is the same than modeling "inheritance"?

i was wondering if there's a difference between modeling a one-to-one relation and a inheritance relation (Table-Per-Type) In both cases we add a foreing key with a UNIQUE constraint, or somethimes the 开发者_JAVA技巧FK is part of the PK

It seems to me that the table structure is the same, but maybe i'm missing something.

Thanks in advance.

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There are a few permutations of this. Here are some: - Suppose A can exist only with B and B can can exist only with A. Then the relationship is one to one. - Suppose A can exist alone and B can extend it, but B cannot exist alone. Then the relationship is inheritance. - Suppose A cannot exist alone, but it can exist with either B or C. Then the relationship is inheritance.

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This is merely a rewording of what others are answering, but I always say the difference is in not in the table structure (which is indeed the same), but in the cardinality constraints on the foreign key:

• in both cases, you have a table T1 with a foreign key F "pointing to" (i.e. containing values from) a key P of another table, T2;
• in both cases, every F points to a different P (P and F are both UNIQUE);
• in both cases, every F actually points to a P (P and F are both non-NULL);
• in case of inheritance, not every P always occurs as a value of F;
• in case of a one-to-one correspondence, every P always occurs as a value of F.

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When it comes to modelling, a one-to-one relationship between entities indicates that they are the same relational table. "The key, the whole key and nothing but the key, so help me Codd!"