我们可以在bothcolumn selection和GROUP BY子句中使用LEAST和GREATEST：

SELECT
    LEAST(col1, col2) AS col1,
    GREATEST(col1, col2) AS col2,
    SUM(col3) AS sum_c
FROM yourtable
GROUP BY 
  LEAST(col1, col2), 
  GREATEST(col1, col2)
ORDER BY col1, col2;

根据您的数据在此sample fiddle上试用。
这里不需要CASE WHEN。

我建议查看分组和排序功能，这是一个如何使用group by和order by来实现您正在尝试做的事情的示例-

SELECT
    CASE WHEN a <= b THEN a ELSE b END AS col1,
    CASE WHEN a <= b THEN b ELSE a END AS col2,
    SUM(c) AS sum_c
FROM your_table
GROUP BY col1, col2
ORDER BY col1, col2;

GROUP BY子句按col1和col2的唯一组合对行进行分组，ORDER BY子句按指定顺序对结果进行排序，CASE语句确保列A和B中的值排序一致，以便将“a b”和“b a”视为相同的组合，最后，SUM函数用于聚合列C中的值