'Creates the SQL for this query. Returns the SQL string and list of
parameters.'

def as_sql(self):

    self.pre_sql_setup()
    if (not self.query.values):
        return ('', ())
    table = self.query.tables[0]
    qn = self.quote_name_unless_alias
    result = [('UPDATE    %s' % qn(table))]
    result.append('SET')
    (values, update_params) = ([], [])
    for (field, model, val) in self.query.values:
        if hasattr(val, 'prepare_database_save'):
            val = val.prepare_database_save(field)
        else:
            val = field.get_db_prep_save(val, connection=self.connection)
        if hasattr(field, 'get_placeholder'):
            placeholder = field.get_placeholder(val, self.connection)
        else:
            placeholder = '%s'
        if hasattr(val, 'evaluate'):
            val = SQLEvaluator(val, self.query, allow_joins=False)
        name = field.column
        if hasattr(val, 'as_sql'):
            (sql, params) = val.as_sql(qn, self.connection)
            values.append(('%s    =    %s' % (qn(name), sql)))
            update_params.extend(params)
        elif (val is not None):
            values.append(('%s    =    %s' % (qn(name), placeholder)))
            update_params.append(val)
        else:
            values.append(('%s    =    NULL' % qn(name)))
    if (not values):
        return ('', ())
    result.append(',    '.join(values))
    (where, params) = self.query.where.as_sql(qn=qn, connection=self.connection)
    if where:
        result.append(('WHERE    %s' % where))
    return ('    '.join(result), tuple((update_params + params)))
