I agree, the definition I provided (not mine) sounds more like an 'index' than a 'key'. I am not familiar with that usage, I am used to 'key' being used as a l

I agree, the definition I provided (not mine) sounds more like an 'index' than a 'key'. I am not familiar with that usage, I am used to 'key' being used as a logical concept, a constraint, while 'index' is a physical concept, some structure that is there 'just' to make shit go faster. Anyways, some sources don't even define the word 'key', but use it to mean 'unique key'. And some other sources (like the one by OP) use it to mean something else (totally not unique), as they recommend that it be 'unique to some degree' And then some others use the definition I provided above. Is that the one meant by OP? I don't know. I any case, I guess OP meant something like "indexes should be highly selective'. Which BTW is a performance tuning advice, I am not sure that that belongs in 'SQL Style Guide'.