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 char(2) ) 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.
For example, the following query finds all the cities that are situated at an attitude of 500ft or higher:
Note: The inheritance hierarchy is a actually a directed acyclic graph.
SELECT name, altitude FROM cities WHERE altitude > 500; name | altitude -----------+---------- Las Vegas | 2174 Mariposa | 1953 (2 rows)
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 | 845Here 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 TABLE.
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