ROWNUM function in SQL is used to return the row number in a result set. It is typically used to limit the number of rows returned in a result set. Here are some common uses of the ROWNUM function:

1. Retrieve the first N rows of data.

SELECT * 
FROM table_name 
WHERE ROWNUM <= N;

1. Retrieve data within a specified range.

SELECT * 
FROM (
    SELECT *, ROWNUM AS rnum 
    FROM table_name
)
WHERE rnum BETWEEN X AND Y;

1. Return the first N unique rows in the result set.

SELECT DISTINCT column_name 
FROM table_name 
WHERE ROWNUM <= N;

It is important to note that ROWNUM is a specific function in the Oracle database and may have different implementations in other databases.