|PostgreSQL Reference Manual - Volume 1 - SQL Language Reference|
by The PostgreSQL Global Development Group
Paperback (6"x9"), 716 pages
RRP £32.00 ($49.95)
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3.3.4 Primary Keys
Technically, a primary key constraint is simply a combination of a unique constraint and a not-null constraint. So, the following two table definitions accept the same data:
CREATE TABLE products ( product_no integer UNIQUE NOT NULL, name text, price numeric );
CREATE TABLE products ( product_no integer PRIMARY KEY, name text, price numeric );
Primary keys can also constrain more than one column; the syntax is similar to unique constraints:
CREATE TABLE example ( a integer, b integer, c integer, PRIMARY KEY (a, c) );
A primary key indicates that a column or group of columns can be used as a unique identifier for rows in the table. (This is a direct consequence of the definition of a primary key. Note that a unique constraint does not, by itself, provide a unique identifier because it does not exclude null values.) This is useful both for documentation purposes and for client applications. For example, a GUI application that allows modifying row values probably needs to know the primary key of a table to be able to identify rows uniquely.
A table can have at most one primary key. (There can be any number of unique and not-null constraints, which are functionally the same thing, but only one can be identified as the primary key.) Relational database theory dictates that every table must have a primary key. This rule is not enforced by PostgreSQL, but it is usually best to follow it.
|ISBN 0954612027||PostgreSQL Reference Manual - Volume 1 - SQL Language Reference||See the print edition|