SELECT (lastname || ', ' || firstname) as fullname
FROM users
WHERE {some condition}

-- result --
    fullname
Anderson, Alex
Baker, Bob
Clark, Carl

SELECT LISTAGG(fullname, '; ') WITHIN GROUP (ORDER BY fullname) as instructors
FROM
(
    SELECT (lastname || ', ' || firstname) as fullname
    FROM users
    WHERE {some condition}
)

-- desired result --
    instructors
Anderson, Alex; Baker, Bob; Clark, Carl

-- actual result --
Error Type: System.Xml.XmlException
Error Message: hexadecimal value 0x00, is an invalid character. Line 1, position 32.

SELECT LISTAGG(fullname, '; ') WITHIN GROUP (ORDER BY fullname) as instructors
FROM
(
    SELECT (pk1 || ', ' || pk1) as fullname    -- pk1 is the users primary key
    FROM users
    WHERE {some condition}
)

-- result --
    instructors
01, 01; 02, 02; 03, 03