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Window functions intro
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@ -5,7 +5,12 @@ sidebar_label: Window Functions
title: Window Functions
---
ClickHouse supports the standard grammar for defining windows and window functions. The following features are currently supported:
Windows functions let you perform calculations across a set of rows that are related to the current row.
Some of the calculations that you can do are similar to those that can be done with an aggregate function, but a window function doesn't cause rows to be grouped into a single output - the individual rows are still returned.
## Standard Window Functions
ClickHouse supports the standard grammar for defining windows and window functions. The table below indicates whether a feature is currently supported.
| Feature | Support or workaround |
|------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
@ -25,6 +30,8 @@ ClickHouse supports the standard grammar for defining windows and window functio
## ClickHouse-specific Window Functions
There are also the following window function that's specific to ClickHouse:
### nonNegativeDerivative(metric_column, timestamp_column[, INTERVAL X UNITS])
Finds non-negative derivative for given `metric_column` by `timestamp_column`.
@ -33,40 +40,6 @@ The computed value is the following for each row:
- `0` for 1st row,
- ${metric_i - metric_{i-1} \over timestamp_i - timestamp_{i-1}} * interval$ for $i_th$ row.
## References
### GitHub Issues
The roadmap for the initial support of window functions is [in this issue](https://github.com/ClickHouse/ClickHouse/issues/18097).
All GitHub issues related to window functions have the [comp-window-functions](https://github.com/ClickHouse/ClickHouse/labels/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
## Syntax
```text
@ -80,20 +53,7 @@ WINDOW window_name as ([[PARTITION BY grouping_column] [ORDER BY sorting_column]
- `PARTITION BY` - defines how to break a resultset into groups.
- `ORDER BY` - defines how to order rows inside the group during calculation aggregate_function.
- `ROWS or RANGE` - defines bounds of a frame, aggregate_function is calculated within a frame.
- `WINDOW` - allows to reuse a window definition with multiple expressions.
### Functions
These functions can be used only as a window function.
- `row_number()` - Number the current row within its partition starting from 1.
- `first_value(x)` - Return the first non-NULL value evaluated within its ordered frame.
- `last_value(x)` - Return the last non-NULL value evaluated within its ordered frame.
- `nth_value(x, offset)` - Return the first non-NULL value evaluated against the nth row (offset) in its ordered frame.
- `rank()` - Rank the current row within its partition with gaps.
- `dense_rank()` - Rank the current row within its partition without gaps.
- `lagInFrame(x)` - Return a value evaluated at the row that is at a specified physical offset row before the current row within the ordered frame.
- `leadInFrame(x)` - Return a value evaluated at the row that is offset rows after the current row within the ordered frame.
- `WINDOW` - allows multiple expressions to use the same window definition.
```text
PARTITION
@ -112,8 +72,23 @@ These functions can be used only as a window function.
└─────────────────┘ <--- UNBOUNDED FOLLOWING (END of the PARTITION)
```
### Functions
These functions can be used only as a window function.
- `row_number()` - Number the current row within its partition starting from 1.
- `first_value(x)` - Return the first non-NULL value evaluated within its ordered frame.
- `last_value(x)` - Return the last non-NULL value evaluated within its ordered frame.
- `nth_value(x, offset)` - Return the first non-NULL value evaluated against the nth row (offset) in its ordered frame.
- `rank()` - Rank the current row within its partition with gaps.
- `dense_rank()` - Rank the current row within its partition without gaps.
- `lagInFrame(x)` - Return a value evaluated at the row that is at a specified physical offset row before the current row within the ordered frame.
- `leadInFrame(x)` - Return a value evaluated at the row that is offset rows after the current row within the ordered frame.
## Examples
Let's have a look at some examples of how window functions can be used.
```sql
CREATE TABLE wf_partition
(
@ -589,6 +564,41 @@ ORDER BY
└──────────────┴─────────────────────┴───────┴─────────────────────────┘
```
## References
### GitHub Issues
The roadmap for the initial support of window functions is [in this issue](https://github.com/ClickHouse/ClickHouse/issues/18097).
All GitHub issues related to window functions have the [comp-window-functions](https://github.com/ClickHouse/ClickHouse/labels/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
## Related Content
- Blog: [Working with time series data in ClickHouse](https://clickhouse.com/blog/working-with-time-series-data-and-functions-ClickHouse)