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count-nulls.md

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Count NULLs

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Description

Part of the data cleaning process involves understanding the quality of your data. NULL values are usually best avoided, so counting their occurrences is a common operation. There are several methods that can be used here:

  • sum(if(<column> is null, 1, 0) - use the IF function to return 1 or 0 if a value is NULL or not respectively, then aggregate.
  • count(*) - count(<column>) - use the different forms of the count() aggregation which include and exclude NULLs.
  • sum(case when x is null then 1 else 0 end) - similar to the IF method, but using a CASE statement instead.
with data as (
  select * from unnest([1, 2, null, null, 5]) as x
)

select
  sum(if(x is null, 1, 0)) with_if,
  count(*) - count(x) with_count,
  sum(case when x is null then 1 else 0 end) with_case
from data
with_if with_count with_case
2 2 2