Chapter 9. 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.

Note: 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.