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 (
state char2
) INHERITS (cities);
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
all the cities that are situated at an attitude of 500ft or higher:
SELECT name, altitude
FROM cities
WHERE altitude > 500;
+----------+----------+
|name | altitude |
+----------+----------+
|Las Vegas | 2174 |
+----------+----------+
|Mariposa | 1953 |
+----------+----------+
On the other hand, to find 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 |
+----------+----------+
Here the *
after cities indicates that the query should
be run over cities and all classes below cities in the
inheritance hierarchy. Many of the commands that we
have already discussed -- select, update and delete --
support this *
notation, as do others, like alter.