.. _contributing_dialect_changes:

Contributing dialect changes
============================

One of the best ways that SQLFluff users can improve SQLFluff for themselves
and others is in contributing dialect changes.

Users will likely know their syntax much better than the regular maintainers
and will have access to an instance of that SQL dialect to confirm changes are
valid SQL in that dialect.

If you can fix your own issues then that's often the quickest way of unblocking
any issues preventing you from using SQLFluff! The maintainers are all
volunteers doing this in our spare time and (like you all I'm sure!), we
only have so much time to work on this.

How SQLFluff reads (or parses) SQL
----------------------------------

SQLFluff has a lexer and parser which is built in a very modular fashion that
is easy to read, understand, and expand on without any core programming skills
or deep knowledge of Python or how SQLFluff operates. For more information see
the :ref:`Architecture Documentation <architecture>`, but will cover that
briefly here to give you enough to start contributing.

We also have a robust Continuous Integration pipeline in GitHub where you can
gain confidence your changes are correct and will not break other SQLFluff
users, even before a regular maintainer reviews the code.

SQLFluff defines the syntax it will used in dialect files (more on this later).
If you look at the `dialect_ansi.py`_ file you will see it has syntax like
this:

.. _`dialect_ansi.py`: https://github.com/sqlfluff/sqlfluff/blob/main/src/sqlfluff/dialects/dialect_ansi.py

.. code-block:: python

   class SelectClauseSegment(BaseSegment):
       """A group of elements in a select target statement."""

       type = "select_clause"
       match_grammar = StartsWith(
           Sequence("SELECT", Ref("WildcardExpressionSegment", optional=True)),
           terminator=OneOf(
               "FROM",
               "WHERE",
               "ORDER",
               "LIMIT",
               "OVERLAPS",
               Ref("SetOperatorSegment"),
           ),
           enforce_whitespace_preceding_terminator=True,
       )

       parse_grammar = Ref("SelectClauseSegmentGrammar")

This says the :code:`SelectClauseSegment` starts with :code:`SELECT` or
:code:`SELECT *` and ends when it encounters a :code:`FROM`, :code:`WHERE`,
:code:`ORDER`...etc. line.

The :code:`match_grammar` is what is used primarily to try to match and parse
the statement. It can be relatively simple (as in this case), to quickly match
just the start and terminating clauses. If that is the case, then a
:code:`parse_grammar` is needed to actually delve into the statement itself
with all the clauses and parts it is made up of. The :code:`parse_grammar`
can be fully defined in the class or, like above example, reference another
class with the definition.

The :code:`match_grammar` is used to quickly identify the start and end of
this block, as parsing can be quite intensive and complicated as the parser
tries various combinations of classes and segments to match the SQL
(particularly optional ones like the :code:`WildcardExpressionSegment` above,
or when there is a choice of statements that could be used).

For some statements a quick match is not needed, and so we can delve straight
into the full grammar definition. In that case the :code:`match_grammar` will
be sufficient and we don't need the optional :code:`parse_grammar`.

Here's another statement, which only uses the :code:`match_grammar` and doesn't
have (or need!) an optional :code:`parse_grammar`:

.. code-block::                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         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