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    <p>In the ANSI standard, <code>on</code> is required for all join types except <code>cross join</code>. This applies to Oracle and most other databases.</p> <p>MySQL is a bit different. This is the syntax diagram in the MySQL <a href="http://dev.mysql.com/doc/refman/5.5/en/join.html" rel="nofollow">documentation</a>:</p> <pre><code>join_table: table_reference [INNER | CROSS] JOIN table_factor [join_condition] | table_reference STRAIGHT_JOIN table_factor | table_reference STRAIGHT_JOIN table_factor ON conditional_expr | table_reference {LEFT|RIGHT} [OUTER] JOIN table_reference join_condition | table_reference NATURAL [{LEFT|RIGHT} [OUTER]] JOIN table_factor </code></pre> <p>That is, MySQL allows the <code>on</code> to be optional for <code>join</code>, but not <code>left outer join</code> or <code>right outer join</code>. MySQL does not support <code>full outer join</code>. And, to make it more confusing <code>cross join</code> is like <code>join</code> and <em>can</em> accept an <code>on</code> clause.</p> <p>And, you should ignore the MySQL extensions. <em>Always</em> use an <code>on</code> clause for left, right, and inner joins. <em>Never</em> use an <code>on</code> clause for <code>cross join</code>. I prefer using these to the <code>natural join</code> (which you don't ask about), because I prefer to be explicit in the columns being used for the join.</p> <p>EDIT:</p> <p>According to <a href="http://www.sqlfiddle.com/#!9/d41d8/270" rel="nofollow">SQL Fiddle</a>, version 5.6 gets an error on <code>full outer join</code> when <code>left outer join</code> works. So this generates an error:</p> <pre><code>select * from (select 1 as a) t1 full outer join (select 2 as b) t2 on t1.a = t2.b; </code></pre> <p>This also generates an error:</p> <pre><code>select * from (select 1 as a) t1 full join (select 2 as b) t2; </code></pre> <p>And MySQL documentation (through 5.7) is all very clear that <code>full join</code> is not supported. I'm not sure why your query might have worked.</p>
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    1. POIs the On Clause Neccessary for Left/Right Outer Joins
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    1. USGordon Linoff
    1. USGordon Linoff
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    1. POIs the On Clause Neccessary for Left/Right Outer Joins
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    1. COThanks for a great and clear explanation. Just one question, you mentioned MySQL doesn't support full outer join, but this works as I mention above "select * from employee full join department;" isn't this a full outer join? Also, I use Mysql 5.6.10 which I think is the latest version; so they might have added this recently.
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      1. USCyrus Rezai
    2. COThe reason the OP's query worked is because they just aliased the `employee` table as `full` :-)
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      1. USMartin Smith
    3. CO@Martin: Great catch.
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