The WEEK scalar function returns the week of the year of the argument as an integer value in range 1-54. The week starts with Sunday.

The schema is SYSFUN.

expression

An expression that returns a value of one of the following built-in data types:
DATE, TIMESTAMP, or a valid character string representation of a date or timestamp that is not a CLOB. In a Unicode database, if a supplied argument is a graphic string, it is first converted to a character string before the function is executed.

The result of the function is INTEGER.
The result can be null; if the argument is null, the result is the null value.