Document multi-row SQL operations with OVER

[sanderson]

SQL supports the OVER clause that allows you to perform operations involving multiple rows. For example, you can do things like moving averages:

SELECT *,
  avg(temp) OVER(ORDER BY time
     ROWS BETWEEN 2 PRECEDING AND CURRENT ROW )
     as moving_average
FROM home
WHERE room = 'Kitchen'

• Update the current OVER clause example to actually use the OVER clause
• Add a query guide with examples of using the OVER clause

Relevant URLs

• https://docs.influxdata.com/influxdb/cloud-dedicated/reference/sql/#over-clause
• https://docs.influxdata.com/influxdb/cloud-dedicated/query-data/sql/