SQL foreign key references

Question

Does

FOREIGN KEY (a) REFERENCES A(a),FOREIGN KEY (b) REFERENCES A(b)

Has the same meanning as in :

FOREIGN KEY (a,b) REFERENCES A(a,b)?

Answer 1

No.

Having two references statement means that both a and b appear in A independently. That is, they are valid values, but they don't have to appear together. Think of "February 30". It has a valid month and a valid day of month, when each are checked separately.

Having a single references statement means that a and b appear in A together. Hence, "February 30" would fail this test.

Traditionally, a foreign keys refer to a primary or unique key. This is not enforced in MySQL, but it is a useful guideline (and also useful in practice). So, in the example with two references, this would usually mean that both a and b are unique. Although this is not enforced, it gives a good indication of how the keys will be used. In the second case, the combination of a and b is unique.