ROW_NUMBER window function
Determines the ordinal number of the current row within a group of rows, counting from 1, based on the ORDER BY expression in the OVER clause. If the optional PARTITION BY clause is present, the ordinal numbers are reset for each group of rows. Rows with equal values for the ORDER BY expressions receive the different row numbers nondeterministically.
Syntax
ROW_NUMBER () OVER ( [ PARTITION BY expr_list ] [ ORDER BY order_list ] )
Arguments
 ( )

The function takes no arguments, but the empty parentheses are required.
 OVER

The window clauses for the ROW_NUMBER function.
 PARTITION BY expr_list

Optional. One or more expressions that define the ROW_NUMBER function.
 ORDER BY order_list

Optional. The expression that defines the columns on which the row numbers are based. If no PARTITION BY is specified, ORDER BY uses the entire table.
If ORDER BY does not produce a unique ordering or is omitted, the order of the rows is nondeterministic. For more information, see Unique ordering of data for window functions.
Return type
BIGINT
Examples
The following example partitions the table by SELLERID and orders each partition by QTY (in ascending order), then assigns a row number to each row. The results are sorted after the window function results are applied.
select salesid, sellerid, qty, row_number() over (partition by sellerid order by qty asc) as row from winsales order by 2,4; salesid  sellerid  qty  row +++ 10006  1  10  1 10001  1  10  2 10005  1  30  3 20001  2  20  1 20002  2  20  2 30001  3  10  1 30003  3  15  2 30004  3  20  3 30007  3  30  4 40005  4  10  1 40001  4  40  2 (11 rows)
For a description of the WINSALES table, see Overview example for window functions.