Impala aggregate functions
Aggregate
functions are a special category with different rules. These functions calculate a return value
across all the items in a result set, so they require a FROM
clause in the
query.
select count(product_id) from product_catalog;
select max(height), avg(height) from census_data where age > 20;
Aggregate functions also ignore NULL
values rather than returning a NULL
result. For example, if some rows have NULL
for a particular column, those rows are
ignored when computing the AVG()
for that column. Likewise, specifying
COUNT(col_name)
in a query counts only those rows where
col_name contains a non-NULL
value.