Filtered aggregates in ClickHouse

ClickHouse provides a simple and intuitive way to write filtered aggregates. For example, compare the standard SQL way to write filtered aggregates (which work fine in ClickHouse) with the shorthand syntax using the -If aggregate function combinator, which can be appended to any aggregate function:

--standard SQL
SELECT
   avg(number)
FILTER (WHERE number > 50)
FROM numbers(100)

--ClickHouse using an aggregate combinator
SELECT
   avgIf(number, number > 50)
FROM numbers(100)

Similarly, there is a -Distinct aggregate combinator:

--standard SQL
SELECT avg(DISTINCT number)

--ClickHouse using an aggregate combinator
SELECT avgDistinct(number)

Why are filtered aggregates are important? Because they allow you to implement the "segment comparison" feature in web analytics services. For example:

WITH
   Region = 'us' AS segment1,
   Browser = 'Chrome' AS segment2
SELECT
   uniqIf(UserID, segment1),
   uniqIf(UserID, segment2)
WHERE segment1 OR segment2

Check out the aggregate function combinator page in the docs for more details.