Inheritance
Let's create two classes. The capitals class contains
state capitals which are also cities. Naturally, the
capitals class should inherit from cities.
CREATE TABLE cities (
name text,
population float,
altitude int -- (in ft)
);
CREATE TABLE capitals UNDER cities (
state char(2)
);
In this case, an instance of capitals inherits all
attributes (name, population, and altitude) from its
parent, cities. The type of the attribute name is
text, a native Postgres type for variable length
ASCII strings. The type of the attribute population is
float, a native Postgres type for double precision
floating point numbers. State capitals have an extra
attribute, state, that shows their state. In Postgres,
a class can inherit from zero or more other classes,
and a query can reference either all instances of a
class or all instances of a class plus all of its
descendants.
The inheritance hierarchy is a actually a directed acyclic graph.
For example, the following query finds the names of all cities,
including state capitals, that are located at an altitude
over 500ft, the query is:
SELECT c.name, c.altitude
FROM cities c
WHERE c.altitude > 500;
which returns:
+----------+----------+
|name | altitude |
+----------+----------+
|Las Vegas | 2174 |
+----------+----------+
|Mariposa | 1953 |
+----------+----------+
|Madison | 845 |
+----------+----------+
On the other hand, the following query finds
all the cities, but not capital cities
that are situated at an attitude of 500ft or higher:
SELECT name, altitude
FROM ONLY cities
WHERE altitude > 500;
+----------+----------+
|name | altitude |
+----------+----------+
|Las Vegas | 2174 |
+----------+----------+
|Mariposa | 1953 |
+----------+----------+
Here the ONLY
before cities indicates that the query should
be run over only cities and not classes below cities in the
inheritance hierarchy. Many of the commands that we
have already discussed -- SELECT,
UPDATE and DELETE --
support this ONLY
notation.
Deprecated
In previous versions of Postgres, the
default was not to get access to child tables. This was found to
be error prone and is also in violation of SQL. Under the old
syntax, to get the sub-classes you append "*" to the table name.
For example
SELECT * from cities*;
To get the old behavior, the set configuration option
SQL_Inheritance to off, e.g.,
SET SQL_Inheritance TO OFF;
or add a line in your postgresql.conf file.