Fixed #24925 -- Document using Coalesce on MySQL

    >>> print(aggregated['combined_age_default'])

    None

.. warning::

    A python value passed to ``Coalesce`` on MySQL may be converted to an

    incorrect type unless explicitly cast to the correct database type:

    >>> from django.db.models.expressions import RawSQL

    >>> from django.utils import timezone

    >>> now = timezone.now()

    >>> now_sql = RawSQL("cast(%s as datetime)", (now,))

    >>> Coalesce('updated', now_sql)

Concat

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