[experimental] Window Functions

ClickHouse supports the standard grammar for defining windows and window functions. The following features are currently supported:

Feature	Support or workaround
ad hoc window specification (`count() over (partition by id order by time desc)`)	supported
expressions involving window functions, e.g. `(count() over ()) / 2)`	not supported, wrap in a subquery (feature request)
`WINDOW` clause (`select … from table window w as (partiton by id)`)	supported
`ROWS` frame	supported
`RANGE` frame	supported, the default
`INTERVAL` syntax for `DateTime` `RANGE OFFSET` frame	not supported, specify the number of seconds instead
`GROUPS` frame	not supported
Calculating aggregate functions over a frame (`sum(value) over (order by time)`)	all aggregate functions are supported
`rank()`, `dense_rank()`, `row_number()`	supported
`lag/lead(value, offset)`	Not supported. Workarounds:
	1) replace with `any(value) over (…. rows between <offset> preceding and <offset> preceding)`, or `following` for `lead`
	2) use `lagInFrame/leadInFrame`, which are analogous, but respect the window frame. To get behavior identical to `lag/lead`, use `rows between unbounded preceding and unbounded following`