5.4.3. Unique Constraints

Unique constraints ensure that the data contained in a column, or a group of columns, is unique among
all the rows in the table. The syntax is:
CREATE TABLE products (
product_no integer UNIQUE,
name text,
price numeric
);
when written as a column constraint, and:
CREATE TABLE products (product_no integer,
name text,
price numeric,
UNIQUE (product_no)
);
when written as a table constraint.
To define a unique constraint for a group of columns, write it as a table constraint with the column
names separated by commas:
CREATE TABLE example (
a integer,
b integer,
c integer,
UNIQUE (a, c)
);
This specifies that the combination of values in the indicated columns is unique across the whole table,
though any one of the columns need not be (and ordinarily isn’t) unique.
You can assign your own name for a unique constraint, in the usual way:
CREATE TABLE products (
product_no integer CONSTRAINT must_be_different UNIQUE,
name text,
price numeric
);
Adding a unique constraint will automatically create a unique B-tree index on the column or group of
columns listed in the constraint. A uniqueness restriction covering only some rows cannot be written
as a unique constraint, but it is possible to enforce such a restriction by creating a unique partial index.
In general, a unique constraint is violated if there is more than one row in the table where the values of
all of the columns included in the constraint are equal. However, two null values are never considered
equal in this comparison. That means even in the presence of a unique constraint it is possible to
store duplicate rows that contain a null value in at least one of the constrained columns. This behavior
conforms to the SQL standard, but we have heard that other SQL databases might not follow this rule.
So be careful when developing applications that are intended to be portable.