Stacks and Queues

Queue Enqueue/Dequeue

Enqueue values at the back and dequeue them from the front in first-in, first-out order.

Algorithm

Basic Implementation

basic.sql 
.mode list
.headers off
CREATE TABLE queue(pos INTEGER PRIMARY KEY, val INTEGER);
INSERT INTO queue(pos, val) VALUES (0, 10), (1, 20), (2, 30);
SELECT GROUP_CONCAT(val, ' -> ')
FROM (SELECT val FROM queue ORDER BY pos ASC);

The queue keeps the oldest value at the front and adds new values at the back.

Step 1 - Enqueue 10, 20, 30

New values join at the back. The oldest value, 10, stays at the front.

Queue after three enqueues: front 10, then 20, then back 30.nextnext10front2030back

Step 2 - Dequeue removes 10

Removing from the front returns 10 and makes 20 the new front.

After one dequeue: removed is 10; front moves to 20.next10removed20front30back

Complexity

  • Time: O(1) per operation with a real queue
  • Space: O(n)

Implementation notes

  • Keep the explicit stack/queue operations. Library shortcuts that only produce the final list hide the data-structure behavior this lesson is meant to replay.
  • The final output uses a deterministic a -> b -> c format for cross-language comparison.
front The front is the oldest value still waiting in the queue.
FIFO A queue removes values in first-in, first-out order.