Oracle repeat field set to null function

An example of using the LAG analysis function to set the repeating field to null is as follows:

The data for the test table test_xx are as follows:

To achieve the following output results:

The test SQL statement is

The code is as follows	Copy Code
SELECT case          when Pre_name is NULL THEN            user_name          ELSE            NULL        end as user_name,        COURSE,        GRADE   from (SELECT A . *,                LAG (user_name) over ( PARTITION by user_name ORDER by COURSE) as Pre_name           from TEST _xx A)