Add Qualify statement to Bigquery

[tuanchris]

-- If this is a parsing or linting issue, please include a minimal SQL example which reproduces the issue, along with the sqlfluff parse output, sqlfluff lint output and sqlfluff fix output when relevant. Finally, please add any labels that are related to the issue before submitting so that it can be addressed easily! --

Expected Behaviour

Qualify should be parsed for BigQuery dialect

Observed Behaviour

found unparsable section: '\n qualify row_num = 1'

Steps to Reproduce

Version

Include the output of sqlfluff --version along with your Python version

sqlfluff, version 0.6.2

Configuration

Include your SQLFluff configuration here

[barrywhart]

Can you provide an example query that produces the issue? Also, if possible, a link to the BigQuery documentation for QUALIFY?

[tuanchris]

@barrywhart Here is the Bigquery Documentation for QUALIFY. A sample query could be:

SELECT
  id,
  ROW_NUMBER() OVER(PARTITION BY id ORDER BY created_date DESC) rownum
FROM `dataset.schema.source_table`
where true 
qualify rownum = 1