SGML source for new documentation.

doc/src/sgml/inherit.sgml

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+<Chapter>
+<Title>Inheritance</Title>
+
+<Para>
+     Let's create two classes. The capitals  class  contains
+     state  capitals  which  are also cities. Naturally, the
+     capitals class should inherit from cities.
+     
+<ProgramListing>
+CREATE TABLE cities (
+    name            text,
+    population      float,
+    altitude        int            -- (in ft)
+);
+
+CREATE TABLE capitals (
+    state           char2
+) INHERITS (cities);
+</ProgramListing>
+
+     In this case, an  instance  of  capitals  <FirstTerm>inherits</FirstTerm>  all
+     attributes  (name,  population,  and altitude) from its
+     parent, cities.  The type  of  the  attribute  name  is
+     <Type>text</Type>,  a  native  <ProductName>Postgres</ProductName>  type  for variable length
+     ASCII strings.  The type of the attribute population is
+     <Type>float</Type>,  a  native <ProductName>Postgres</ProductName> type for double precision
+     floating point numbers.  State capitals have  an  extra
+     attribute, state, that shows their state.  In <ProductName>Postgres</ProductName>,
+     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>
+<Para>
+The inheritance hierarchy is a actually a directed acyclic graph.
+</Para>
+</Note>
+For example, the  following  query  finds
+     all  the cities that are situated at an attitude of 500ft or higher:
+     
+<ProgramListing>
+SELECT name, altitude
+    FROM cities
+    WHERE altitude &gt; 500;
+
++----------+----------+
+|name      | altitude |
++----------+----------+
+|Las Vegas | 2174     |
++----------+----------+
+|Mariposa  | 1953     |
++----------+----------+
+</ProgramListing>         
+
+<Para>
+     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:
+
+<ProgramListing>
+SELECT c.name, c.altitude
+    FROM cities* c
+    WHERE c.altitude > 500;
+</ProgramListing>
+
+     which returns:
+     
+<ProgramListing>
++----------+----------+
+|name      | altitude |
++----------+----------+
+|Las Vegas | 2174     |
++----------+----------+
+|Mariposa  | 1953     |
++----------+----------+
+|Madison   | 845      |
++----------+----------+
+</ProgramListing>
+
+     Here the <Quote>*</Quote> 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 -- <Command>select</Command>, <Command>update</Command> and <Command>delete</Command> --
+     support this <Quote>*</Quote> notation, as do others, like <Command>alter</Command>.
+</Para>
+
+</Chapter>