lead

Description

Return the value of one column in a following row across an ordered group. offset is the number of rows forward from the current row to obtain a value. If no ordering is specified, the default ordering is that of the invoking verb, which is generally the valid_from timestamp, ascending. In this case, lead(col, 1) returns the next value for column col.

Return type

storable value

Domain

This is a window function (calculates over a group of multiple input rows using windowing.)

Categories

• Window

Usage

lead(expression, leadby)

Argument	Type	Optional	Repeatable	Restrictions
expression	storable value	no	no	none
leadby	int64	no	no	constant

Examples

make_col next_value:window(lead(value, 1), group_by(key))

Obtain the next value within the group identified by key and store the results in a new column named next_value. Because we did not specify a order_by clause in the window function nor in the verb, the lead function would use the default ordering, which is time ascending.

timestamp	key	value	next_value
100	A	5	4
150	B	6	5
180	A	4	3
300	B	5	null
400	A	3	null

make_col low_value:window(lead(value, 2), order_by(value, false), group_by(key))

Obtain the second lowest value above the current value within the group identified by key and store the results in a new column named low_value. We use an order_by function to order the rows by value ascending.

timestamp	key	value	low_value
100	A	5	null
200	B	6	null
200	A	4	7
300	B	5	null
400	A	3	5
400	B	2	6
500	A	7	null

make_col next_value:window(lead(value, 1), group_by(key), frame_exact(ahead:100ns))

Obtain the next value within the group identified by key and store the results in a new column named next_value. Because a frame_exact is specified for the window function, next_value would be null if the most recent value is found outside the window frame.

timestamp	key	value	next_value
100	A	5	4
150	B	6	null
180	A	4	null
300	B	5	null
400	A	3	null