7.4 interactive docs.
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<chapter id="ddl">
<title>Data Definition</title>
evaluated whenever the default value is inserted
(<emphasis>not</emphasis> when the table is created). A common example
is that a timestamp column may have a default of <literal>now()</>,
- so that it gets set to the time of row insertion.
+ so that it gets set to the time of row insertion. Another common
+ example is generating a <quote>serial number</> for each row.
+ In <productname>PostgreSQL</productname> this is typically done by
+ something like
+<programlisting>
+CREATE TABLE products (
+ product_no integer <emphasis>DEFAULT nextval('products_product_no_seq')</emphasis>,
+ ...
+);
+</programlisting>
+ where the <literal>nextval()</> function supplies successive values
+ from a <firstterm>sequence object</> (see <xref
+ linkend="functions-sequence">). This arrangement is sufficiently common
+ that there's a special shorthand for it:
+<programlisting>
+CREATE TABLE products (
+ product_no <emphasis>SERIAL</emphasis>,
+ ...
+);
+</programlisting>
+ The <literal>SERIAL</> shorthand is discussed further in <xref
+ linkend="datatype-serial">.
</para>
</sect1>
</para>
+ <para>
+ A table can inherit from more than one parent table, in which case it has
+ the union of the columns defined by the parent tables (plus any columns
+ declared specifically for the child table).
+ </para>
+
<para>
A serious limitation of the inheritance feature is that indexes (including
unique constraints) and foreign key constraints only apply to single
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<sect1 id="history">
<listitem>
<para>
- In addition to the monitor program, a new program
+ A new program
(<application>psql</application>) was provided for interactive
SQL queries, which used <acronym>GNU</acronym>
- <application>Readline</application>.
+ <application>Readline</application>. This largely superseded
+ the old <application>monitor</> program.
</para>
</listitem>
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<chapter id="tutorial-sql">
<programlisting>
SELECT * FROM weather;
</programlisting>
- (here <literal>*</literal> means <quote>all columns</quote>).
+ Here <literal>*</literal> is a shorthand for <quote>all columns</quote>.
<footnote>
<para>
While <literal>SELECT *</literal> is useful for off-the-cuff
since adding a column to the table would change the results.
</para>
</footnote>
+ So the same result would be had with:
+<programlisting>
+SELECT city, temp_lo, temp_hi, prcp, date FROM weather;
+</programlisting>
+
The output should be:
<screen>
</para>
<para>
- You may specify any arbitrary expressions in the select list. For
- example, you can do:
+ You can write expressions, not just simple column references, in the
+ select list. For example, you can do:
<programlisting>
SELECT city, (temp_hi+temp_lo)/2 AS temp_avg, date FROM weather;
</programlisting>
</para>
<para>
- Arbitrary Boolean operators (<literal>AND</literal>,
+ A query can be <quote>qualified</> by adding a <literal>WHERE</>
+ clause that specifies which rows are wanted. The <literal>WHERE</>
+ clause contains a Boolean (truth value) expression, and only rows for
+ which the Boolean expression is true are returned. The usual
+ Boolean operators (<literal>AND</literal>,
<literal>OR</literal>, and <literal>NOT</literal>) are allowed in
- the qualification of a query. For example, the following
+ the qualification. For example, the following
retrieves the weather of San Francisco on rainy days:
<programlisting>
SELECT * FROM weather
- WHERE city = 'San Francisco'
- AND prcp > 0.0;
+ WHERE city = 'San Francisco' AND prcp > 0.0;
</programlisting>
Result:
<screen>
<para>
<indexterm><primary>ORDER BY</primary></indexterm>
+
+ You can request that the results of a query
+ be returned in sorted order:
+
+<programlisting>
+SELECT * FROM weather
+ ORDER BY city;
+</programlisting>
+
+<screen>
+ city | temp_lo | temp_hi | prcp | date
+---------------+---------+---------+------+------------
+ Hayward | 37 | 54 | | 1994-11-29
+ San Francisco | 43 | 57 | 0 | 1994-11-29
+ San Francisco | 46 | 50 | 0.25 | 1994-11-27
+</screen>
+
+ In this example, the sort order isn't fully specified, and so you
+ might get the San Francisco rows in either order. But you'd always
+ get the results shown above if you do
+
+<programlisting>
+SELECT * FROM weather
+ ORDER BY city, temp_lo;
+</programlisting>
+ </para>
+
+ <para>
<indexterm><primary>DISTINCT</primary></indexterm>
<indexterm><primary>duplicate</primary></indexterm>
- As a final note, you can request that the results of a query can
- be returned in sorted order or with duplicate rows removed:
+ You can request that duplicate rows be removed from the result of
+ a query:
<programlisting>
SELECT DISTINCT city
- FROM weather
- ORDER BY city;
+ FROM weather;
</programlisting>
<screen>
(2 rows)
</screen>
- <literal>DISTINCT</literal> and <literal>ORDER BY</literal> can be
- used separately, of course.
+ Here again, the result row ordering might vary.
+ You can ensure consistent results by using <literal>DISTINCT</literal> and
+ <literal>ORDER BY</literal> together:
+ <footnote>
+ <para>
+ In some database systems, including older versions of
+ <productname>PostgreSQL</productname>, the implementation of
+ <literal>DISTINCT</literal> automatically orders the rows and
+ so <literal>ORDER BY</literal> is redundant. But this is not
+ required by the SQL standard, and current
+ <productname>PostgreSQL</productname> doesn't guarantee that
+ <literal>DISTINCT</literal> causes the rows to be ordered.
+ </para>
+ </footnote>
+
+<programlisting>
+SELECT DISTINCT city
+ FROM weather
+ ORDER BY city;
+</programlisting>
</para>
</sect1>
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<chapter id="tutorial-start">
<para>
Possibly, your site administrator has already created a database
for your use. He should have told you what the name of your
- database is. In this case you can omit this step and skip ahead
+ database is. In that case you can omit this step and skip ahead
to the next section.
</para>
</para>
<para>
- If you do not have the privileges required to create a database,
- you will see the following:
+ Another response could be this:
+<screen>
+createdb: could not connect to database template1: FATAL: user "joe" does not
+exist
+</screen>
+ where your own login name is mentioned. This will happen if the
+ administrator has not created a <productname>PostgreSQL</> user account
+ for you. (<productname>PostgreSQL</> user accounts are distinct from
+ operating system user accounts.) If you are the administrator, see
+ <xref linkend="user-manag"> for help creating accounts. You will need to
+ become the operating system user under which <productname>PostgreSQL</>
+ was installed (usually <literal>postgres</>) to create the first user
+ account. It could also be that you were assigned a
+ <productname>PostgreSQL</> user name that is different from your
+ operating system user name; in that case you need to use the <option>-U</>
+ switch or set the <envar>PGUSER</> environment variable to specify your
+ <productname>PostgreSQL</> user name.
+ </para>
+
+ <para>
+ If you have a user account but it does not have the privileges required to
+ create a database, you will see the following:
<screen>
createdb: database creation failed: ERROR: permission denied to create database
</screen>
</para>
<para>
- If you have encountered problems starting <command>psql</command>
+ If you encounter problems starting <command>psql</command>
then go back to the previous section. The diagnostics of
- <command>psql</command> and <command>createdb</command> are
- similar, and if the latter worked the former should work as well.
+ <command>createdb</command> and <command>psql</command> are
+ similar, and if the former worked the latter should work as well.
</para>
<para>
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