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WITH mytable AS (
SELECT 1 id, 'x' a, 'y' b, 'r' c
UNION ALL SELECT 2, 'x', 'y', 'q'
UNION ALL SELECT 3, 'x', 'y', 'f'
)
SELECT a, b, GROUP_CONCAT(c ORDER BY id) c
FROM mytable
GROUP BY a, b
ORDER BY a, b
| a | b | c |
|---|---|---|
| x | y | r,q,f |