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    <ol> <li><p>The compiler is not confused.</p> <p>The error is simply alerting you to the fact that the aggregate functions don't make sense in that context:</p> <pre><code>having count(col2) = max(count(col2) </code></pre> <p>The HAVING clause takes effect after the GROUP BY, so <code>COUNT(COL2)</code> is the count of non-null values in the <code>COL2</code> column for each distinct value of <code>col1</code>. In the context of one distinct value of <code>col1</code>, <code>MAX(COUNT(COL2))</code> makes no sense - if the result of <code>COUNT(COL2)</code> is 3, then <code>MAX(3)</code>, obviously, is 3 - but here the compiler is smart enough to know that you probably didn't intend it that way.</p> <p>So, what you were intending is that the <code>COUNT(COL2)</code> on the left-hand side was supposed to be for that distinct value of <code>COL1</code>), but the <code>COUNT(COL2)</code> on the right-hand side was supposed to be over all values of <code>COL1</code>. The expression as a whole, therefore, is a mixture and is not valid SQL.</p></li> <li><p>Yes, you can think of it that way if you like. The deepest subquery is run first, which returns a result set (in this case, a single row with a single column) to the calling subquery.</p></li> </ol> <p>You may find the following query is more efficient, instead of running effectively two queries against the table:</p> <pre><code>where t1.col1 = (select col1 from (select col1, count_col2, MAX(count_col2) OVER () max_count_col2 from (select col1, count(col2) AS count_col2 from t2 GROUP BY col1)) where count_col2 = max_count_col2) </code></pre>
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    1. POHow does ORA-00935 trigger? Seeking insight on Oracle SQL's nested group function methodology
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    1. USJeffrey Kemp
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    1. COInteresting stuff, I'll take a glance at OVER, I haven't seen that before. As for efficiency, probably; I'm wrapping up a SQL 101 course, and am having code to some fairly specific constraints. Regardless, I thank you for your time in answering.
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