Use SYSTEM_USER to retrieve the current active user within the host operating system.
Example:

          SELECT SYSTEM_USER AS 'SysUser';

Result:

> PostgreSQL, SQL Server, and Sybase support SYSTEM_USER.

> Access, DB2, MySQL, and Oracle don’t support SYSTEM_USER.

Use SESSION_USER to retrieve the current active Authorization ID, if it differs from the USER.
Example:

          SELECT SESSION_USER AS 'AuthID';

Result:

> PostgreSQL, SQL Server, and Sybase support SESSION_USER.

> Access, DB2, MySQL, and Oracle don’t support SESSION_USER.

Use CURRENT_USER or USER to retrieve the current active user of the computer where the DBMS is running.

Example:

     SELECT CURRENT_USER AS 'User';

Result:

> DB2, MySQL, PostgreSQL, SQL Server, and Sybase support CURRENT_USER.

> Access doesn’t support CURRENT_USER. Instead it uses CURRENTUSER.

> DB2 uses SELECT CURRENT_USER FROM SYSIBM.SYSDUMMY1;.

> Oracle doesn’t support CURRENT_USER, but it does support SELECT USER FROM DUAL;.

> CURRENT_USER and USER are synonymous.