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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
IFfunction 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