The Firebird 5.0 Language Reference | Navigational Functions

`LEAD()`

Value from a row in the current partition with a given offset after the current row

Result type

The same as type as expr

Syntax

LEAD ( <expr> [, <offset [, <default>]])
  OVER <window_name_or_spec>

Table 1. Arguments of `LEAD`
Argument	Description
expr	Expression.May contain a table column, constant, variable, expression, scalar function.Aggregate functions are not allowed as an expression.
offset	The offset in rows after the current row to get the value identified by expr.If offset is not specified, the default is `1`.offset can be a column, subquery or other expression that results in a positive integer value, or another type that can be implicitly converted to `BIGINT`.`offset` cannot be negative (use `LAG` instead).
default	The default value to return if offset points outside the partition.Default is `NULL`.

The LEAD function provides access to the row in the current partition with a given offset after the current row.

If offset points outside the current partition, default will be returned, or NULL if no default was specified.

`NTH_VALUE()`

The Nth value starting from the first or the last row of the current frame

Result type

The same as type as expr

Syntax

NTH_VALUE ( <expr>, <offset> )
  [FROM {FIRST | LAST}]
  OVER <window_name_or_spec>

Table 1. Arguments of `NTH_VALUE`
Argument	Description
expr	Expression.May contain a table column, constant, variable, expression, scalar function.Aggregate functions are not allowed as an expression.
offset	The offset in rows from the start (`FROM FIRST`), or the last (`FROM LAST`) to get the value identified by expr.offset can be a column, subquery or other expression that results in a positive integer value, or another type that can be implicitly converted to `BIGINT`.`offset` cannot be zero or negative.

The NTH_VALUE function returns the Nth value starting from the first (FROM FIRST) or the last (FROM LAST) row of the current frame, see also note on frame for navigational functions.Offset 1 with FROM FIRST is equivalent to FIRST_VALUE, and offset 1 with FROM LAST is equivalent to LAST_VALUE.

Aggregate Functions Inside Window Specification

It is possible to use aggregate functions (but not window functions) inside the OVER clause.In that case, first the aggregate function is applied to determine the windows, and only then the window functions are applied on those windows.

Note	When using aggregate functions inside `OVER`, all columns not used in aggregate functions must be specified in the `GROUP BY` clause of the `SELECT`.

Using an Aggregate Function in a Window Specification

select
  code_employee_group,
  avg(salary) as avg_salary,
  rank() over (order by avg(salary)) as salary_rank
from employee
group by code_employee_group