First Non-Repeating Value

Find the first input value whose final frequency is one.

Algorithm

Canonical input [3, 5, 2, 5, 3, 8, 2] prints 8. The replay uses the same input in every language, so this SQL DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.sql

WITH input(pos, value) AS (
  VALUES (1, 3), (2, 5), (3, 2), (4, 5), (5, 3), (6, 8), (7, 2)
),
counts AS (
  SELECT value, COUNT(*) AS freq FROM input GROUP BY value
)
SELECT input.value
FROM input JOIN counts USING (value)
WHERE counts.freq = 1
ORDER BY input.pos
LIMIT 1;

Complexity

Time: O(n) average
Space: O(k) for k distinct values

Implementation notes

Keep output formatting deterministic. Do not rely on unordered hash-map printing when the lesson needs cross-language comparison.
The trace highlights the hash table state after each write.

two-pass lookup The first pass builds a frequency table. The second pass keeps the original order and stops at the first value with frequency one.