[dragonwriter]

> Unique and never changes can be two things concatenated,

No, for a good primary key, the "unique" parts can be due to concatenation, but the whole key (and thus all the elements) needs to be unchanging. [0]

[0] Modern databases can actually deal with changeable PKs, though with a potentially serious performance hit, but in many use cases where data travels outside the database for some kind of interaction where the results need to get reentered into the database, this is still a problem, since you can't (in any general way) cascade updates to things (which may not even be online systems, e.g., paper records feeding human processes) outside of the DB.