Update nested field in BigQuery table

Question

I am trying to perform what, you would think, is a trivial operation in BigQuery; I am trying to update a nested field in a BigQuery table that is the result of a 360 export.

Here is my query:

#standardSQL
UPDATE `dataset_name`.`ga_sessions_20170705`
SET hits.eventInfo.eventLabel = 'some string'
WHERE TRUE

But I get this error message:

Error: Cannot access field eventInfo on a value with type ARRAY<STRUCT<item STRUCT<transactionId INT64, currencyCode STRING>, isEntrance BOOL, minute INT64, ...>> at [3:10]

How can I update this nested field?

Answer 1

hits is an array, so you need to use an array subquery to assign to it. It would look something like this:

#standardSQL
UPDATE `dataset_name`.`ga_sessions_20170705`
SET hits =
  ARRAY(
    SELECT AS STRUCT * REPLACE(
      (SELECT AS STRUCT eventInfo.* REPLACE('some string' AS eventLabel)) AS eventInfo)
    FROM UNNEST(hits)
  )
WHERE TRUE;

Answer 2

If you need to modify a given custom dimension you can use this:

#standardSQL
UPDATE `tablename`
SET hits = 
  ARRAY(
    SELECT AS STRUCT * REPLACE(
      ARRAY(
        SELECT AS STRUCT cd.index,
          CASE WHEN cd.index = index_number THEN 'new value'
          ELSE cd.value
          END
        FROM UNNEST(customDimensions) AS cd
      ) AS customDimensions)
    FROM UNNEST(hits) hit
  )
WHERE TRUE

But it takes a while to run.