BigQuery SQL: Cannot query the cross product of repeated fields /

Question

My goal is to filter from all visitors, to only analyse customers (which are in the customDimension.index =2 and then further filter only specific types of pageviews for the customers.

SELECT customDimensions.value AS CustomerID,
SUM(totals.pageviews) as page_views,
SUM(CASE WHEN hits.type = 'PAGE' AND hits.contentGroup.contentGroup2 = 'important' THEN 1 ELSE 0 END) AS important_pageviews
FROM `xxxxxxxx.ga_sessions_20180415`
WHERE customDimensions.index = 2
GROUP BY CustomerID 

I get the error that (using StandardSQL):

Error: Cannot access field index on a value with type
ARRAY<STRUCT<index INT64, value STRING>> at [5:24]

For Legacy SQL:

Error: Cannot query the cross product of repeated fields customDimensions.index and hits.contentGroup.contentGroup2.

Edit:

SELECT cd.value AS CustomerID,
SUM(totals.pageviews) as page_views,
SUM(CASE WHEN hits.type = 'PAGE' AND hits.contentGroup.contentGroup2 = 'important' THEN 1 ELSE 0 END) AS important_pageviews
FROM `xxxxxxxx.ga_sessions_20180415`,
UNNEST(customDimensions) AS cd
WHERE cd.index = 2
GROUP BY CustomerID 

returns:

Error: Cannot access field type on a value with type ARRAY<STRUCT<hitNumber INT64, time INT64, hour INT64, ...>> at [3:20]

I tried correcting the line 3:20 using UNNEST(hits.type) = 'PAGE' AND UNNEST(hitscontentGroup.contentGroup2) = 'important' which gives Error: Syntax error: Unexpected keyword UNNEST at [3:15]

Answer 1

As customDimensions is an array you'll need to unnest this in order to refer to it's contents, see the StandardSQL example below where I unnest UserIDs from Google Analytics data in BigQuery:

SELECT customDimension.value AS UserID
FROM `my.project.data` AS t
  CROSS JOIN UNNEST(t.customdimensions) AS customDimension
  WHERE customDimension.index = 2