--- title: "first_value(), nth_value(), last_value()" url: "https://docs.yugabyte.com/stable/api/ysql/exprs/window_functions/function-syntax-semantics/first-value-nth-value-last-value/" --- # first_value(), nth_value(), last_value() Describes the functionality of the YSQL window functions first_value(), nth_value(), and last_value(). These three window functions fall into the second group, [Window functions that return column(s) of another row within the window](/stable/api/ysql/exprs/window_functions/function-syntax-semantics/#window-functions-that-return-column-s-of-another-row-within-the-window "Window functions that return column(s) of another row within the window") in the section [List of all window functions](/stable/api/ysql/exprs/window_functions/function-syntax-semantics/#list-of-all-window-functions "List of all window functions"). Each of the functions in the second group makes obvious sense when the scope within which the specified row is found is the entire [*window*](/stable/api/ysql/exprs/window_functions/invocation-syntax-semantics/#the-window-definition-rule "window"). Only this use will be described here. When used this way, each of these functions, as their names suggest, return the same result for each row in the current [*window*](/stable/api/ysql/exprs/window_functions/invocation-syntax-semantics/#the-window-definition-rule "window"). See [Examples that illustrate all three functions](#examples-that-illustrate-all-three-functions "Examples that illustrate all three functions") below. If you have a use case that requires a specifically tailored [*window frame*](/stable/api/ysql/exprs/window_functions/invocation-syntax-semantics/#frame-clause-semantics-for-window-functions "window frame"), then see the section [The frame\_clause](/stable/api/ysql/exprs/window_functions/invocation-syntax-semantics/#the-frame-clause "The frame_clause"). ## first\_value() **Signature:** ``` input value: anyelement return value: anyelement ``` **Purpose:** Return the specified value from the first row, in the specified sort order, in the current [*window frame*](/stable/api/ysql/exprs/window_functions/invocation-syntax-semantics/#frame-clause-semantics-for-window-functions "window frame"). If you specify the [`frame_clause`](/stable/api/ysql/syntax_resources/grammar_diagrams/#frame-clause "frame_clause") to start at a fixed offset before the current row, then `first_value()` would produce the same result as would the correspondingly parameterized `lag()`. If this is your aim, then you should use `lag()` for clarity. ## nth\_value() **Signature:** ``` input value: anyelement, int return value: anyelement ``` **Purpose:** Return the specified value from the "*Nth"* row, in the specified sort order, in the current [*window frame*](/stable/api/ysql/exprs/window_functions/invocation-syntax-semantics/#frame-clause-semantics-for-window-functions "window frame"). The second, mandatory, parameter specifies *"N"* in *"Nth"*. ## last\_value() **Signature:** ``` input value: anyelement return value: anyelement ``` **Purpose:** Return the specified value from the last row, in the specified sort order, in the current [*window frame*](/stable/api/ysql/exprs/window_functions/invocation-syntax-semantics/#frame-clause-semantics-for-window-functions "window frame"). ## Examples that illustrate all three functions If you haven't yet installed the tables that the code examples use, then go to the section [The data sets used by the code examples](/stable/api/ysql/exprs/window_functions/function-syntax-semantics/data-sets/ "The data sets used by the code examples"). This example uses table *"t1"*. Notice that it has been contrived so that the last *"v"* (ordered by *"k"*) for each value of *"class"* is `NULL`. Use the technique shown in the section [Using `nth_value()` and `last_value()` to return the whole row](/stable/api/ysql/exprs/window_functions/function-syntax-semantics/#using-nth-value-and-last-value-to-return-the-whole-row "Using nth_value() and last_value() to return the whole row") so that each of the three window functions produces all of the fields in each row: ```plpgsql drop type if exists rt cascade; create type rt as (class int, k int, v int); select class, k, first_value((class, k, v)::rt::text) over w as fv, nth_value ((class, k, v)::rt::text, 3) over w as nv, last_value ((class, k, v)::rt::text) over w as lv from t1 window w as ( partition by class order by k range between unbounded preceding and unbounded following); ``` Here is the result. To make it easier to see the pattern, a break has been manually inserted here between each successive set of rows with the same value for *"class"*. ``` class | k | fv | nv | lv -------+----+-----------+-----------+--------- 1 | 1 | (1,1,1) | (1,3,3) | (1,5,) 1 | 2 | (1,1,1) | (1,3,3) | (1,5,) 1 | 3 | (1,1,1) | (1,3,3) | (1,5,) 1 | 4 | (1,1,1) | (1,3,3) | (1,5,) 1 | 5 | (1,1,1) | (1,3,3) | (1,5,) 2 | 6 | (2,6,6) | (2,8,8) | (2,10,) 2 | 7 | (2,6,6) | (2,8,8) | (2,10,) 2 | 8 | (2,6,6) | (2,8,8) | (2,10,) 2 | 9 | (2,6,6) | (2,8,8) | (2,10,) 2 | 10 | (2,6,6) | (2,8,8) | (2,10,) 3 | 11 | (3,11,11) | (3,13,13) | (3,15,) 3 | 12 | (3,11,11) | (3,13,13) | (3,15,) 3 | 13 | (3,11,11) | (3,13,13) | (3,15,) 3 | 14 | (3,11,11) | (3,13,13) | (3,15,) 3 | 15 | (3,11,11) | (3,13,13) | (3,15,) 4 | 16 | (4,16,16) | (4,18,18) | (4,20,) 4 | 17 | (4,16,16) | (4,18,18) | (4,20,) 4 | 18 | (4,16,16) | (4,18,18) | (4,20,) 4 | 19 | (4,16,16) | (4,18,18) | (4,20,) 4 | 20 | (4,16,16) | (4,18,18) | (4,20,) 5 | 21 | (5,21,21) | (5,23,23) | (5,25,) 5 | 22 | (5,21,21) | (5,23,23) | (5,25,) 5 | 23 | (5,21,21) | (5,23,23) | (5,25,) 5 | 24 | (5,21,21) | (5,23,23) | (5,25,) 5 | 25 | (5,21,21) | (5,23,23) | (5,25,) ``` Notice that the `::text` typecast of a *"row"* type value renders `NULL` simply as an absence. This explains why you see, for example, *"(1,5,)"* for each value produced by `last_value()` in the [*window*](/stable/api/ysql/exprs/window_functions/invocation-syntax-semantics/#the-window-definition-rule "window") where *"k=1"*. This basic example certainly demonstrates the meaning of *"first"*, *"Nth"* (for *"N=3"*), and *"last"*. But it isn't very useful because, just as these names suggest, the output is the same for each row in a particular [*window*](/stable/api/ysql/exprs/window_functions/invocation-syntax-semantics/#the-window-definition-rule "window"). The following query adds a conventional `GROUP BY` clause. It also extracts the interesting fields from the *"row"* type value that each window function produces as individual values. ```plpgsql drop type if exists rt cascade; create type rt as (class int, k int, v int); \pset null '??' with a as ( select class, first_value((class, k, v)::rt) over w as fv, nth_value ((class, k, v)::rt, 3) over w as nv, last_value ((class, k, v)::rt) over w as lv from t1 window w as ( partition by class order by k range between unbounded preceding and unbounded following)) select (fv).class as fv_class, (fv).k as fv_k, (fv).v as fv_v, (nv).k as nv_k, (nv).v as nv_v, (lv).k as lv_k, (lv).v as lv_v from a group by (fv).class, (fv).k, (fv).v, (nv).k, (nv).v, (lv).k, (lv).v order by 1; ``` This is the result: ``` fv_class | fv_k | fv_v | nv_k | nv_v | lv_k | lv_v ----------+------+------+------+------+------+------ 1 | 1 | 1 | 3 | 3 | 5 | ?? 2 | 6 | 6 | 8 | 8 | 10 | ?? 3 | 11 | 11 | 13 | 13 | 15 | ?? 4 | 16 | 16 | 18 | 18 | 20 | ?? 5 | 21 | 21 | 23 | 23 | 25 | ?? ```