avg

SELECT avg(x) FROM VALUES('x Int8', 0, 1, 2, 3, 4, 5);

┌─avg(x)─┐
│    2.5 │
└────────┘

CREATE TABLE test (t UInt8) ENGINE = Memory;

SELECT avg(t) FROM test;

┌─avg(t)─┐
│    nan │
└────────┘

CREATE TABLE requests (req_id Int64, duration IntervalMillisecond) ENGINE = Memory;

INSERT INTO requests VALUES (1, 100), (2, 200), (3, 300);

SELECT avg(duration) AS avg_duration, toTypeName(avg_duration) FROM requests;

┌─avg_duration─┬─toTypeName(avg_duration)─┐
│          200 │ IntervalMillisecond      │
└──────────────┴──────────────────────────┘