BCNF is not used for cases where a relation has

Not every relation can be decomposed into BCNF without cost. A dependency-preserving, lossless-join decomposition into 3NF is always achievable, but the same guarantee does not extend to BCNF -- a BCNF decomposition is always lossless-join, yet it can fail to preserve every functional dependency. This gap shows up specifically when a relation's candidate keys overlap, i.e. two or more candidate keys share at least one common attribute.

Consider a relation R(A, B, C) with functional dependencies AB → C and C → B. Here AB and AC are both candidate keys, and they overlap on A. R is in 3NF (C is part of a candidate key), but not in BCNF (C → B violates BCNF since C alone is not a superkey). Decomposing R into BCNF forces splitting off {C, B}; the dependency AB → C can then no longer be checked without rejoining the tables, so dependency preservation is lost. This overlapping-candidate-key situation is exactly the case the question describes.

None of the other listed conditions creates this conflict on its own:

• Two (or more) candidate keys — having several candidate keys is ordinary, and as long as they do not overlap, it causes no dependency-preservation problem.

• Two candidate keys and composite — a candidate key spanning multiple attributes (a composite key) is likewise unremarkable on its own; the difficulty needs the keys to also share an attribute.

• Two mutually exclusive foreign keys — foreign keys govern references between relations, not a relation's own candidate-key structure, so they play no role in this BCNF limitation.