left join去重

左连接（left join）不会自动去重，它会返回左表中的所有记录，同时匹配右表中的记录。如果需要去重，可以使用group by语句结合聚合函数来实现。在左连接的结果集上使用group by可以对group的记录进行去重，并且可以计算出总分。例如，可以使用以下查询语句实现左连接去重：

select g.group_id, g.group_name, group_concat(m.member_name) as member_names, sum(s.score) as sum_score 
from groups g 
left join members m on g.group_id = m.group_id 
left join score s on s.member_id = m.member_id 
where m.is_quit = 0 or m.is_quit is null 
group by g.group_id;

这个查询语句会返回每个小组的唯一记录，并且会将小组成员的姓名拼接成一个字符串，并计算出总分。
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