PostgreSQL
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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 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.

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