Categories:

Window function syntax and usage (Ranking)

LEAD¶

Accesses data in a subsequent row in the same result set without having to join the table to itself.

See also:

LAG

Syntax¶

LEAD ( <expr> [ , <offset> , <default> ] ) [ { IGNORE | RESPECT } NULLS ]
  OVER ( [ PARTITION BY <expr1> ] ORDER BY <expr2> [ { ASC | DESC } ] )
Copy

Arguments¶

expr

The string expression to be returned.

offset

The number of rows forward from the current row from which to obtain a value. For example, an offset of 2 returns the expr value with an interval of 2 rows.

Note that setting a negative offset has the same effect as using the LAG function.

Default is 1. If IGNORE NULLS is specified, maximum is 1,000,000.

default

The expression to return when the offset goes out of the bounds of the window. Supports any expression whose type is compatible with expr.

Default is NULL.

{ IGNORE | RESPECT } NULLS

Whether to ignore or respect NULL values when an expr contains NULL values:

  • IGNORE NULLS excludes any row whose expression evaluates to NULL when offset rows are counted.

  • RESPECT NULLS includes any row whose expression evaluates to NULL when offset rows are counted.

Default: RESPECT NULLS

Usage notes¶

  • The PARTITION BY clause partitions the result set produced by the FROM clause into partitions to which the function is applied. For more information, see Window function syntax and usage.

  • The ORDER BY clause orders the data within each partition.

Examples¶

CREATE OR REPLACE TABLE sales(
  emp_id INTEGER,
  year INTEGER,
  revenue DECIMAL(10,2));

INSERT INTO sales VALUES
  (0, 2010, 1000),
  (0, 2011, 1500),
  (0, 2012, 500),
  (0, 2013, 750);
INSERT INTO sales VALUES
  (1, 2010, 10000),
  (1, 2011, 12500),
  (1, 2012, 15000),
  (1, 2013, 20000);
INSERT INTO sales VALUES
  (2, 2012, 500),
  (2, 2013, 800);

SELECT emp_id,
       year,
       revenue,
       LEAD(revenue) OVER (PARTITION BY emp_id ORDER BY year) - revenue AS diff_to_next
  FROM sales
  ORDER BY emp_id, year;
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+--------+------+----------+--------------+
| EMP_ID | YEAR |  REVENUE | DIFF_TO_NEXT |
|--------+------+----------+--------------|
|      0 | 2010 |  1000.00 |       500.00 |
|      0 | 2011 |  1500.00 |     -1000.00 |
|      0 | 2012 |   500.00 |       250.00 |
|      0 | 2013 |   750.00 |         NULL |
|      1 | 2010 | 10000.00 |      2500.00 |
|      1 | 2011 | 12500.00 |      2500.00 |
|      1 | 2012 | 15000.00 |      5000.00 |
|      1 | 2013 | 20000.00 |         NULL |
|      2 | 2012 |   500.00 |       300.00 |
|      2 | 2013 |   800.00 |         NULL |
+--------+------+----------+--------------+

CREATE OR REPLACE TABLE t1 (
  c1 NUMBER,
  c2 NUMBER);

INSERT INTO t1 VALUES
  (1,5),
  (2,4),
  (3,NULL),
  (4,2),
  (5,NULL),
  (6,NULL),
  (7,6);

SELECT c1,
       c2,
       LEAD(c2) IGNORE NULLS OVER (ORDER BY c1)
  FROM t1;
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+----+------+------------------------------------------+
| C1 | C2   | LEAD(C2) IGNORE NULLS OVER (ORDER BY C1) |
|----+------+------------------------------------------|
|  1 |  5   |                                        4 |
|  2 |  4   |                                        2 |
|  3 | NULL |                                        2 |
|  4 |  2   |                                        6 |
|  5 | NULL |                                        6 |
|  6 | NULL |                                        6 |
|  7 |  6   |                                     NULL |
+----+------+------------------------------------------+
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