inherit.sgml

来自「关系型数据库 Postgresql 6.5.2」· SGML 代码 · 共 89 行

<Chapter Id="inherit">
<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>

<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>