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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML
><HEAD
><TITLE
>The Query Language</TITLE
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><H1
><A
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>Chapter 12. The Query Language</A
></H1
><DIV
CLASS="TOC"
><DL
><DT
><B
>Table of Contents</B
></DT
><DT
><A
HREF="query-ug.html#AEN2847"
>Concepts</A
></DT
><DT
><A
HREF="query-ug2863.html"
>Creating a New Class</A
></DT
><DT
><A
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>Populating a Class with Instances</A
></DT
><DT
><A
HREF="query-ug2896.html"
>Querying a Class</A
></DT
><DT
><A
HREF="query-ug2913.html"
>Redirecting SELECT Queries</A
></DT
><DT
><A
HREF="query-ug2920.html"
>Joins Between Classes</A
></DT
><DT
><A
HREF="query-ug2935.html"
>Updates</A
></DT
><DT
><A
HREF="query-ug2939.html"
>Deletions</A
></DT
><DT
><A
HREF="query-ug2946.html"
>Using Aggregate Functions</A
></DT
></DL
></DIV
><P
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>This chapter must go into depth on each area of the query language.
Currently a copy of the tutorial.
- thomas 1998-01-12</P
></BLOCKQUOTE
></P
><P
> The <SPAN
CLASS="PRODUCTNAME"
>Postgres</SPAN
> query language is a variant of
<SPAN
CLASS="ACRONYM"
>SQL3</SPAN
>. It
has many extensions such as an extensible type system,
inheritance, functions and production rules. Those are
features carried over from the original <SPAN
CLASS="PRODUCTNAME"
>Postgres</SPAN
>
query
language, <SPAN
CLASS="PRODUCTNAME"
>PostQuel</SPAN
>.
This section provides an overview
of how to use <SPAN
CLASS="PRODUCTNAME"
>Postgres</SPAN
> <SPAN
CLASS="ACRONYM"
>SQL</SPAN
>
to perform simple operations.
This manual is only intended to give you an idea of our
flavor of <SPAN
CLASS="ACRONYM"
>SQL</SPAN
> and is in no way a complete tutorial on
<SPAN
CLASS="ACRONYM"
>SQL</SPAN
>. Numerous books have been written on <SPAN
CLASS="ACRONYM"
>SQL</SPAN
>. For
instance, consult <A
HREF="biblio.html#MELT93"
><I
>Understanding the New <I
>SQL</I
></I
></A
> or
<A
HREF="biblio.html#DATE97"
><I
>A Guide to the <I
>SQL</I
> Standard</I
></A
>. You should also
be aware that some features of <SPAN
CLASS="PRODUCTNAME"
>Postgres</SPAN
>
are not part of the <SPAN
CLASS="ACRONYM"
>ANSI</SPAN
> standard.</P
><H1
CLASS="SECT1"
><A
NAME="AEN2847"
>Concepts</A
></H1
><P
> The fundamental notion in <SPAN
CLASS="PRODUCTNAME"
>Postgres</SPAN
>
is that of a class,
which is a named collection of object instances. Each
instance has the same collection of named attributes,
and each attribute is of a specific type. Furthermore,
each instance has a permanent <I
CLASS="FIRSTTERM"
>object
identifier</I
> (<SPAN
CLASS="ACRONYM"
>OID</SPAN
>)
that is unique throughout the installation. Because
<SPAN
CLASS="ACRONYM"
>SQL</SPAN
> syntax refers to tables, we will use the terms
<I
CLASS="FIRSTTERM"
>table</I
> and <I
CLASS="FIRSTTERM"
>class</I
> interchangeably.
Likewise, an <SPAN
CLASS="ACRONYM"
>SQL</SPAN
> <I
CLASS="FIRSTTERM"
>row</I
> is an
<I
CLASS="FIRSTTERM"
>instance</I
> and <SPAN
CLASS="ACRONYM"
>SQL</SPAN
>
<I
CLASS="FIRSTTERM"
>columns</I
>
are <I
CLASS="FIRSTTERM"
>attributes</I
>.
As previously discussed, classes are grouped into
databases, and a collection of databases managed by a
single <TT
CLASS="FILENAME"
>postmaster</TT
> process constitutes an installation
or site.</P
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