SELECT DISTINCT or GROUP BY: Which MySQL Query is Faster for Unique Values?

Performance comparison of SELECT DISTINCT and GROUP BY in MySQL

In MySQL, you can use SELECT DISTINCT or GROUP BY query to extract unique values ​​in the table. However, there is some ambiguity as to which method performs better.

Consider the following form:

<code class="language-sql">CREATE TABLE users (
  id int(10) unsigned NOT NULL auto_increment,
  name varchar(255) NOT NULL,
  profession varchar(255) NOT NULL,
  employer varchar(255) NOT NULL,
  PRIMARY KEY  (id)
)</code>

To retrieve unique values ​​for the "profession" field, you can use two queries:

Query 1 (SELECT DISTINCT):

<code class="language-sql">SELECT DISTINCT u.profession FROM users u</code>

Query 2 (GROUP BY):

<code class="language-sql">SELECT u.profession FROM users u GROUP BY u.profession</code>

Which one is faster?

Although these two queries appear functionally equivalent, their performance may vary depending on the query optimizer. Typically, SELECT DISTINCT is slightly faster.

This is because GROUP BY groups by all visible columns by default. In this case, because no other columns are used in the query, the optimizer may need to perform additional work to identify whether the grouping does not make sense. On the other hand, SELECT DISTINCT explicitly requests a unique value.

Conclusion

In most cases, the performance difference between SELECT DISTINCT and GROUP BY is negligible. However, if optimization is critical, it is recommended to use SELECT DISTINCT as it simplifies the task of the query optimizer.

Tip: To obtain conclusive results, it is recommended to always test both methods in a performance monitoring environment.

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