SQL with ambiguous column
<p>Check the following example (Oracle syntax):</p>

<pre><code>create table t1 (id number, a varchar2(255));
create table t2 (id number, a varchar2(255));
create table t3 (id number);

select * from t1 where id in (select id from t2 where a = 'aa');
select * from t1 where id in (select id from t3 where a = 'aa');
</code></pre>

<p>Both select work fine but they use different attributes for filtering. In my opinion first SQL should give an error because column a is ambiguously defined. Is this not defined in the official SQL standard? </p>