Linked List in SQL

Columns of table "linked_list":
unique_id integer (unique)
list_id integer (indexed)
next_id : integer (FK unique_id)
value : ???

List [A, B] with id 42:
(1, 42, 2, A)
(2, 42, null, B)

List [ x,y,z ] with id 123:
(3, 123, 4, x)
(4, 123, 5, y)
(5, 123, null, z)

Now I insert new value Q between x and y in list 123:
UPDATE: (3, 123, 6, x)
INSERT: (6, 123, 4, Q)

You know that 1 and 3 are both heads because no other row links to them.
You know 2 and 5 are last items because they reference null.
An empty list simply has no rows.