mysql 重复数据，求和过滤的处理

第一种按照分组求和就可以了
SELECT ID, SUM(NUM)
FROM tab
GROUP BY ID

第二种
SELECT tab.ID, t2.NUM
FROM tab LEFT JOIN (SELECT ID, SUM(NUM) AS NUM FROM tab GROUP BY ID) t2 ON tab.ID = t2.ID

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问题一：查询出某个字段所有不重复的数据以及求和？
问题二：查询出所有不重复的数据？
问题三：查询数据中所有某字段重复的记录？
id        name           address

1         a                 shanghai
2         a                 beijing
3         b                 hongkong
4         c                 hunan
5         d                 hubei

解决问题一：
1）、distinct

select distinct name from tablename;--求数据
select count(distinct name) from tablename;--求和
2）、group by
select name from tablename group by name;--求数据
select count(min(id)) from tablename group by name;--求和

select count(max(id)) from tablename group by name;--求和
解决问题二：
select * from tablename where id in(select min(id) from tablenamegroup by name) order by id;
select * from tablename where id in(select max(id)from tablename group by name) order by id;

解决问题三：
select *
from table
where name in
(select name from table group by name having count(name) >1);
注：having count(name) >1 表示相同name列数据出现的次数，大于1就表示重复数据。