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SELECT
a.id id_a,
a.color color_a,
b.id id_b,
b.color color_b
FROM
palette_a a
RIGHT JOIN palette_b b ON a.color = b.color
WHERE a.id IS NULL;
xxxxxxxxxx
SELECT
a.id id_a,
a.color color_a,
b.id id_b,
b.color color_b
FROM
palette_a a
RIGHT JOIN palette_b b ON a.color = b.color;