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Rule: Do Not Use `DISTINCT` with Parentheses
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**Rule Code:** ``ST08``

**Name:** ``distinct``

Overview
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In SQL, the `DISTINCT` keyword is used to remove duplicate rows from the result set. However, `DISTINCT` should not be used with parentheses, as it can cause confusion and is not part of the SQL standard. Using parentheses with `DISTINCT` can lead to incorrect assumptions about how the query works and may even result in errors in some databases. Instead, `DISTINCT` should be applied to the entire set of selected columns without parentheses.

Explanation
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Anti-pattern: Using `DISTINCT` with Parentheses
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Using parentheses after `DISTINCT` is an anti-pattern because it is not part of the SQL standard and can lead to incorrect interpretations of the query. Some developers mistakenly assume that `DISTINCT(a)` only applies the `DISTINCT` keyword to the column `a`, but `DISTINCT` always applies to all the columns in the `select` list.

**Example of `DISTINCT` with Parentheses (Anti-pattern):**

.. code-block:: sql

    select distinct(a), b
      from my_table;

In this example, `DISTINCT(a)` incorrectly suggests that the `DISTINCT` keyword only applies to column `a`, but in reality, the parentheses are unnecessary and can cause confusion.

Best Practice: Use `DISTINCT` Without Parentheses
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To ensure clarity and adherence to the SQL standard, `DISTINCT` should be used without parentheses and should apply to all selected columns in the query. This improves the readability and correctness of the SQL query.

**Refactored Example Without Parentheses (Best Practice):**

.. code-block:: sql

    select distinct
           a,
           b
      from my_table;

In this refactored example, `DISTINCT` is applied correctly without parentheses, ensuring that the query behaves as expected by removing duplicate rows based on both `a` and `b` columns.

Conclusion
----------

Avoiding the use of parentheses with the `DISTINCT` keyword ensures that SQL queries are both correct and adherent to the SQL standard. Following this rule improves readability and prevents confusion, ensuring that the `DISTINCT` keyword applies to all selected columns in the query.

Groups:
-------

- `all <../..>`_
- `structure <../..#structure-rules>`_