What is the MAP keyword in SQL?
<p>I have been trying to improve my SQL chops a little bit by working through <a href="http://sqlkoans.com/" rel="nofollow">SQL Koans</a>, since I enjoy the learn-by-doing-and-meditating approach, and my SQL knowledge is lacking. In one set of koans is the following:</p>

<pre><code>-- Meditate on MANY-TO-MANY relationships
select a.first_name, a.last_name, b.title
from book b
    join book_to_author_map map on _____.id = _____.book_id
    join author a on _____.author_id = _____.id
where author_id in (1, 5, 6)
</code></pre>

<p>Having no previous experience with aliasing, and little experience with joins, I was stuck on this problem for a little while. I was stuck longer than necessary though, because in Emacs <code>sql-mode</code> the word <strong><code>map</code></strong> (which I understand now to be an alias for the table <code>book_to_author_map</code>) <strong>is highlighted as an SQL keyword</strong>. I spent a lot of time looking for documentation on this keyword, and found nothing (aside from lots of information on <a href="http://sqlmap.org/" rel="nofollow">sqlmap</a>...). </p>

<p>Peeking at the source code for the Emacs <code>sql-mode</code> I found that <code>map</code> is designated as a keyword as part of <code>sql-mode-postgres-font-lock-keywords</code>, so I started to search for <code>map</code> in relation to PostgreSQL, and found it in <a href="https://www.postgresql.org/docs/8.3/static/sql-keywords-appendix.html" rel="nofollow">a list of SQL keywords in the PostgreSQL documentation</a>. The keyword <code>MAP</code> is designated as a "non-reserved" keyword for SQL:2003 and a "reserved" keyword for SQL:1999. However, I have been unable so far to find any documentation for this keyword in association with SQL. </p>

<p>My question, more out of curiosity than anything else, is as in the title: <strong>what is the <code>MAP</code> keyword in SQL?</strong></p>