Stacks and Queues

Queue Enqueue/Dequeue

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

Algorithm

The replay uses the same three values in every language, so this SQL DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.sql 
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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);

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.