How Does MySQL's `GROUP BY` Clause Behave Without Aggregate Functions?

MySQL's GROUP BY Clause: Understanding Non-Aggregate Queries

MySQL's GROUP BY clause, when used without aggregate functions in the SELECT statement, exhibits unique behavior. While seemingly allowing the omission of columns from the grouping, this functionality relies on a crucial assumption: the omitted columns must possess identical values within each group.

Indeterminate Results: The Pitfalls of Omission

If the omitted columns contain varying values within a group, the results become unpredictable. MySQL selects arbitrary values from each group for the omitted columns. This means the output isn't consistently reliable.

MySQL's Optimization and its Implications

MySQL's optimization techniques sometimes permit the omission of columns in GROUP BY to boost performance. However, this optimization depends on the aforementioned assumption of constant values within each group. Violating this assumption leads to the indeterminate results discussed above.

Ensuring Reliable GROUP BY Queries

For predictable and reliable results, it's best practice to explicitly include all relevant columns in the GROUP BY clause. Alternatively, if summarizing data is the goal, use aggregate functions (like MAX, MIN, AVG, SUM, etc.) to obtain deterministic results.

A More Deterministic Approach

To avoid the ambiguity of implicit GROUP BY behavior, consider this alternative query:

<code class="language-sql">SELECT A.*
FROM emp AS A
WHERE A.salary = (
  SELECT MAX(B.salary)
  FROM emp B
  WHERE B.dept = A.dept
);</code>

This approach directly selects rows based on the maximum salary within each department, providing a clear and consistent outcome.

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