group by要聚合函数，不是有函数就行
获取指定客户每月的消费额
https://www.nowcoder.com/practice/ed04f148b63e469e8f62e051d06a46f5
select
left(t.t_time, 7) time,
sum(t.t_amount) total
from
trade t
left join customer c on t.t_cus = c.c_id
where
left(t.t_time, 4) = '2023'
and c.c_name = 'Tom'
and t.t_type = '1'
group by
1 # group by要聚合函数，不是有函数就行
# 如果后面有having，内容只能是group by后的内容，否则不需要having
order by
1 asc
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