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3.3.3 Unique Constraints

Unique constraints ensure that the data contained in a column or a group of columns is unique with respect to 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.

If a unique constraint refers to a group of columns, the columns are listed 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
);

In general, a unique constraint is violated when there are two or more rows in the table where the values of all of the columns included in the constraint are equal. However, two null values are not 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 may not follow this rule. So be careful when developing applications that are intended to be portable.