How to find last item in a repeated structure in bigquery

问题

I have a nested repeated structure, the repeated structure is of variable length. For example, it could be a person object with a repeated structure that holds cities the person has lived in. I'd like to find the last item in that list say to find current city person lives in. Is there an easy way to do this, I tried looking around jsonpath functions but I'm not sure how to use it with "within". Any help please?

回答1:

1) You can use LAST and WITHIN

SELECT  
  FIRST(cell.value) within record ,
  LAST(cell.value) within record 
FROM [publicdata:samples.trigrams] 
where ngram = "! ! That"

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2) or if you want something more advanced you can use POSITION

POSITION(field) - Returns the one-based, sequential position of field within a set of repeated fields.

You can check the samples from trigrams (click on Details to see the unflatten schema) https://bigquery.cloud.google.com/table/publicdata:samples.trigrams?pli=1

And when you run POSITION, you get the ordering of that field.

SELECT  
  ngram,
  cell.value,
  position(cell.volume_count) as pos,
FROM [publicdata:samples.trigrams] 
where ngram = "! ! That"

Now that you have the position, you can query for last one.

来源：https://stackoverflow.com/questions/28557636/how-to-find-last-item-in-a-repeated-structure-in-bigquery