[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

References 

GitHub Issues 

The roadmap for the initial support of window functions is in this issue.

All GitHub issues related to window funtions have the comp-window-functions tag.

Tests 

These tests contain the examples of the currently supported grammar:

https://github.com/ClickHouse/ClickHouse/blob/master/tests/performance/window_functions.xml

https://github.com/ClickHouse/ClickHouse/blob/master/tests/queries/0_stateless/01591_window_functions.sql

Postgres Docs 

https://www.postgresql.org/docs/current/sql-select.html#SQL-WINDOW

https://www.postgresql.org/docs/devel/sql-expressions.html#SYNTAX-WINDOW-FUNCTIONS

https://www.postgresql.org/docs/devel/functions-window.html

https://www.postgresql.org/docs/devel/tutorial-window.html

MySQL Docs 

https://dev.mysql.com/doc/refman/8.0/en/window-function-descriptions.html

https://dev.mysql.com/doc/refman/8.0/en/window-functions-usage.html

https://dev.mysql.com/doc/refman/8.0/en/window-functions-frames.html

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