Unique Index vs. Unique Constraint

Question

Hi All,
I'm studying for the Oracle SQL Expert Certification. At one point in the book, while talking about indices, the author says that a unique index is not the same a unique constraint. However, he doesn't explain why they're two different things.
Could anyone clarify the difference between the two, please?
Thanks a lot,
Valerio

Answer 1

A constraint has different meaning to an index. It gives the optimiser more information and allows you to have foreign keys on the column, whereas a unique index doesn't.
eg:
SQL> create table t1 (col1 number, col2 varchar2(20), constraint t1_uq unique (col1));
Table created.
SQL> create table t2 (col1 number, col2 varchar2(20));
Table created.
SQL> create unique index t2_idx on t2 (col1);
Index created.
SQL> create table t3 (col1 number, col2 number, col3 varchar2(20), constraint t3_fk
  2                   foreign key (col2) references t1 (col1));
Table created.
SQL> create table t4 (col1 number, col2 number, col3 varchar2(20), constraint t4_fk
  2                   foreign key (col2) references t2 (col1));
                 foreign key (col2) references t2 (col1))
ERROR at line 2:
ORA-02270: no matching unique or primary key for this column-listIt's like saying "What's the difference between a car seat and an armchair? They both allow you to sit down!"