# List of window functions in YQL

The syntax for calling window functions is detailed in a separate article.

## Aggregate functions

All aggregate functions can also be used as window functions.
In this case, each row includes an aggregation result obtained on a set of rows from the window frame.

Examples:

SELECT
SUM(int_column) OVER w1 AS running_total,
SUM(int_column) OVER w2 AS total,
FROM my_table
WINDOW
w1 AS (ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW),
w2 AS ();


## ROW_NUMBER

Row number within a partition. No arguments.

Signature

ROW_NUMBER()->Uint64


Examples

SELECT
ROW_NUMBER() OVER w AS row_num
FROM my_table
WINDOW w AS (ORDER BY key);


## LAG / LEAD

Accessing a value from a row in the section that lags behind (LAG) or leads (LEAD) the current row by a fixed number. The first argument specifies the expression to be accessed, and the second argument specifies the offset in rows. You may omit the offset. By default, the neighbor row is used: the previous or next, respectively (hence, 1 is assumed by default). For the rows having no neighbors at a given distance (for example LAG(expr, 3) NULL is returned in the first and second rows of the section.

Signature

LEAD(T[,Int32])->T?
LAG(T[,Int32])->T?


Examples

SELECT
int_value - LAG(int_value) OVER w AS int_value_diff
FROM my_table
WINDOW w AS (ORDER BY key);


## FIRST_VALUE / LAST_VALUE

Access values from the first and last rows (using the ORDER BY clause for the window) of the window frame. The only argument is the expression that you need to access.

Optionally, OVER can be preceded by the additional modifier IGNORE NULLS. It changes the behavior of functions to the first or last non-empty (i.e., non-NULL) value among the window frame rows. The antonym of this modifier is RESPECT NULLS: it's the default behavior that can be omitted.

Signature

FIRST_VALUE(T)->T?
LAST_VALUE(T)->T?


Examples

SELECT
FIRST_VALUE(my_column) OVER w
FROM my_table
WINDOW w AS (ORDER BY key);

SELECT
LAST_VALUE(my_column) IGNORE NULLS OVER w
FROM my_table
WINDOW w AS (ORDER BY key);


## RANK / DENSE_RANK

Number the groups of neighboring rows in the section that have the same expression value in the argument. DENSE_RANK numbers the groups one-by-one, and RANK skips (N - 1) values, with N being the number of rows in the previous group.

If there is no argument, it uses the order specified in the ORDER BY section in the window definition.
If the argument is omitted and ORDER BY is not specified, then all rows are considered equal to each other.

Note

Passing an argument to RANK/DENSE_RANK is a non-standard extension in YQL.

Signature

RANK([T])->Uint64
DENSE_RANK([T])->Uint64


Examples

SELECT
RANK(my_column) OVER w
FROM my_table
WINDOW w AS (ORDER BY key);

SELECT
RANK() OVER w
FROM my_table
WINDOW w AS (ORDER BY my_column);



## SessionState()

A non-standard window function SessionState() (without arguments) lets you get the session calculation status from SessionWindow for the current row.
It's allowed only if SessionWindow() is present in the PARTITION BY section in the window definition.