SQL - PostgreSQL - Count NULLs.md

SQL - PostgreSQL - Count NULLs

Source: sql-snippets/count-nulls.md at main · count/sql-snippets (github.com)

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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:

• 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) - create a new column which contains 1 or 0 depending on whether the value is NULL, then aggregate.

with data as (
  select * from (values (1), (2), (null), (null), (5)) as data (x)
)

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

with_count	with_case
2	2

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