In MySQL does a UNIQUE varchar have to be a PRIMARY KEY as well?

Question

I have 2 unique values under 1 table in MySQL.

|id (PK)|varchar(255) UNIQUE|
-----------------------------
|   1   |      1234abcde    |
|   2   |      5678fghij    |
|   3   |      9012klmno    |

Do I have to set UNIQUE varchar as a PK or I can still use auto_incremented id (int)? Is this a good or bad practice?

Answer 1

No, it doesn't have to be a primary. Primaries are unique, but not every unique is a primary.

Answer 2

One table can have multiple keys. One of them is "primary" (PRIMARY KEY) and the rest are "alternate" (UNIQUE constraint).

But beware that under InnoDB primary key also acts as a clustered index, so choosing one key or another to be primary can have profound performance implications.