Is using multiple unique constrains on a single table considered bad design?

Question

I was looking at PostgreSQL's INSERT INTO .. ON CONFLICT (..) DO UPDATE .. syntax and realized, you cannot do multiple unique constraint checks with it. I mean, you either refer to a composite unique index by the column names ON CONFLICT (Name, Symbol) (if the unique index is defined for these two columns), or you use the primary key. If you define two separate unique indexes for the columns, you can only check for one.

CREATE TABLE student
    (Id int primary key, Name varchar(50), Symbol varchar(50),
      CONSTRAINT col1_unique UNIQUE (Name),
      CONSTRAINT col2_unique UNIQUE (Symbol)
    ); 

INSERT INTO student
    (Id, Name, Symbol)
VALUES
    (1, 'John', 'J'),
    (2, 'David', 'D'),
    (3, 'Will', 'W');

INSERT INTO student
    (Id, Name, Symbol)
VALUES
    (4, 'Jeremy', 'J')
   on conflict(Name) DO UPDATE
   set Name = 'Jeremy';

Could throw an error, saying J is a duplicate. However, this example is simply a bad design, because the Symbol should be in another table and be connected to the student table via a one to many relationship. Which is why I am wondering, maybe PostgreSQL's on conflict was designed this way, because you can ALWAYS restructure the tables in a way, where there is only a single unique index. Is it true or there is an another reason?

Example fiddle: http://www.sqlfiddle.com/#!17/9c0ce

Answer 1

There is always Sixth, or Domain Key, Normal Form. Here each non-key column becomes it own table. So 3NF table T(Key, Col1, Col2, ..) becomes T1(Key, Col1), T2(Key, Col2) etc. Those new tables which require uniqueness can have it declared.

I think having multiple unique constraints on a table is perfectly OK, however. Take for example a table of countries. This would have, say, an ID, the name, the ISO code, the capital city, and some others. Each of those first four will be unique. Moreover, if we want our system to rely on each being unique I believe we should define unique constraints on each. This enforces truths about the data on which all consumers can rely.