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lead

The lead() window function takes a column and an integer offset as arguments, and returns the value of the cell in the column located at the specified number of rows after the current row. It can be used with all data types supported by Redpanda SQL

Syntax

The syntax for this function is:

LEAD (expression, offset, default)
OVER (
    [PARTITION BY partition_expression, ... ]
    ORDER BY sort_expression [ASC | DESC], ...
)

The function returns a value of the same type as the input. If no row meets the offset criteria, the function returns the specified default value, which must be of a type compatible with the input.

Parameters

  • expression: Column to reference.

  • offset: Optional. Number of rows ahead of the current row. Defaults to 1.

  • default: Optional. Value to return if the offset is out of range. Defaults to NULL.

Examples

The following examples use the winsales table that stores details about some sales transactions:

CREATE TABLE winsales(
    salesid int,
    dateid date,
    sellerid int,
    buyerid text,
    qty int,
    qty_shipped int);
INSERT INTO winsales VALUES
    (30001, '8/2/2003', 3, 'b', 10, 10),
    (10001, '12/24/2003', 1, 'c', 10, 10),
    (10005, '12/24/2003', 1, 'a', 30, null),
    (40001, '1/9/2004', 4, 'a', 40, null),
    (10006, '1/18/2004', 1, 'c', 10, null),
    (20001, '2/12/2004', 2, 'b', 20, 20),
    (40005, '2/12/2004', 4, 'a', 10, 10),
    (20002, '2/16/2004', 2, 'c', 20, 20),
    (30003, '4/18/2004', 3, 'b', 15, null),
    (30004, '4/18/2004', 3, 'b', 20, null),
    (30007, '9/7/2004', 3, 'c', 30, null);

Lead(expression, offset)

This example executes the lead() function with expression and offset parameters' values specified:

SELECT buyerid, dateid, qty,
  LEAD(qty,1) OVER (ORDER BY buyerid, dateid) AS next_qty
FROM winsales WHERE buyerid = 'c'
ORDER BY buyerid, dateid;

This query returns the buyer ID, date ID, quantity and previous quantity for all rows with buyer ID equal to c:

  buyerid |   dateid   | qty | next_qty
---------+------------+-----+----------
 c       | 2003-12-24 |  10 |       10
 c       | 2004-01-18 |  10 |       20
 c       | 2004-02-16 |  20 |       30
 c       | 2004-09-07 |  30 |
(4 rows)

Expression, offset and default specified

This example executes the lead() function with expression, offset and default parameters' values specified:

SELECT buyerid, dateid, qty
  LEAD(buyerid,1,'unknown') OVER (ORDER BY dateid) AS next_buyerid
FROM winsales
ORDER BY dateid;

The query returns the buyer ID, date ID, quantity and following buyer ID for all rows:

  buyerid |   dateid   | qty | next_buyerid
---------+------------+-----+--------------
 b       | 2003-08-02 |  10 | c
 c       | 2003-12-24 |  10 | a
 a       | 2003-12-24 |  30 | a
 a       | 2004-01-09 |  40 | c
 c       | 2004-01-18 |  10 | b
 b       | 2004-02-12 |  20 | a
 a       | 2004-02-12 |  10 | c
 c       | 2004-02-16 |  20 | b
 b       | 2004-04-18 |  15 | b
 b       | 2004-04-18 |  20 | c
 c       | 2004-09-07 |  30 | unknown
(11 rows)

Time series: lead() to compare next day’s sales quantity

This example uses lead() to compare each day’s sales quantity (qty) with the next day’s quantity, ordered by dateid:

SELECT dateid, qty,
  LEAD(qty) OVER (ORDER BY dateid) AS next_day_qty,
  LEAD(qty) OVER (ORDER BY dateid) - qty AS qty_change
FROM winsales
ORDER BY dateid;

The query returns:

   dateid   | qty | next_day_qty | qty_change
------------+-----+--------------+------------
 2003-08-02 |  10 |           10 |          0
 2003-12-24 |  10 |           30 |         20
 2003-12-24 |  30 |           40 |         10
 2004-01-09 |  40 |           10 |        -30
 2004-01-18 |  10 |           20 |         10
 2004-02-12 |  20 |           10 |        -10
 2004-02-12 |  10 |           20 |         10
 2004-02-16 |  20 |           15 |         -5
 2004-04-18 |  15 |           20 |          5
 2004-04-18 |  20 |           30 |         10
 2004-09-07 |  30 |              |
(11 rows)