PRIMARY KEY vs. UNIQUE restrictions in MySQL

The distinction between a PRIMARY KEY constraint and a UNIQUE constraint, according to the MySQL documentation, is that a PK constraint does not permit NULL values whereas a UQ constraint does. One may generate both PK and UQ for the same column in a MySQL table!

What use does it serve to add a UNIQUE constraint to a column that already has a PK constraint?
Why won't MySQL let me add a unique constraint to a column with a PK constraint already?



Also, I want to ask according to internet definitions the join condition in equi join is equality (=), whereas inner join can contain additional operators such as less than () or larger than (>). A non-equi join is a sort of join in which the join condition does not employ equals. Does this imply that non-equi joins and inner joins are the same?

In SQL language, NULL simply means "unknown value" or "not yet set value". Because a primary key is intended to (unique) identify a record in a table, a NULL PK has no sense, so is not permitted.