List of window functions in YQL

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

Aggregate functions

All the 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. Without 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 the partition row that is behind (LAG) or ahead (LEAD) of the current partition row by a fixed number of rows. 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) in the first and second rows of the partition), NULL is returned.

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);

SELECT item, odd, LAG(item, 1) OVER w as lag1 FROM (
    SELECT item, item % 2 as odd FROM (
        SELECT AsList(1, 2, 3, 4, 5, 6, 7) as item
    )
    FLATTEN BY item
)
WINDOW w As (
    PARTITION BY odd
    ORDER BY item
);

/* Output:
item  odd  lag1
--------------------
2  0  NULL
4  0  2
6  0  4
1  1  NULL
3  1  1
5  1  3
7  1  5
*/

FIRST_VALUE / LAST_VALUE

Accessing the values from the first and last rows of the window border (in the order of the window's ORDER BY). 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);

NTH_VALUE

Accessing values from the specified row within the window frame (in the order of the window's ORDER BY). The arguments are the expression to access and the row number starting with 1.

Optionally, OVER can be preceded by the additional modifier IGNORE NULLS, which lets you skip rows with NULL in the first argument's value. The antonym of this modifier is RESPECT NULLS: it's the default behavior that can be omitted.

Signature

NTH_VALUE(T,N)->T?

Examples

SELECT
   NTH_VALUE(my_column, 2) OVER w
FROM my_table
WINDOW w AS (ORDER BY key);

SELECT
   NTH_VALUE(my_column, 3) IGNORE NULLS OVER w
FROM my_table
WINDOW w AS (ORDER BY key);

RANK / DENSE_RANK / PERCENT_RANK

Number the groups of neighboring partition rows having 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. PERCENT_RANK returns the relative rank of the current row: (RANK - 1)/(number of rows in the partition - 1).

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.

Signature

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

Examples

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

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

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

NTILE

Distributes the rows of an ordered partition into a specified number of groups. The groups are numbered starting with 1. For each row, the NTILE function returns the number of the group the row belongs to.

Signature

NTILE(Uint64)->Uint64

Examples

SELECT
    NTILE(10) OVER w AS group_num
FROM my_table
WINDOW w AS (ORDER BY key);

CUME_DIST

Returns the relative position (> 0 and <= 1) of a row within a partition. Without arguments.

Signature

CUME_DIST()->Double

Examples

SELECT
    CUME_DIST() OVER w AS dist
FROM my_table
WINDOW w AS (ORDER BY key);

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.