mysql一些实用的语句

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1、计算不重复的数据
select count(distinct(userID)) from relationship;

2、计算相同的数据
select count(1) from relationship group by userID;

3、对同一个表进行查询和删除

delete from relationship where userID in (select * from(select userID from relationship group by userID having count(1)<10)w);

4、根据查询出的结果更新表的内容
UPDATE dataall INNER JOIN user ON dataall.userID=user.userID SET dataall.hash_userID=user.hash_userID;

5、将查询出的内容写入到文件中
select hash_userID,hash_bookmarkID,hash_tagID from dataall WHERE NO%4!=0 into outfile ‘D:\dataall201010base1.txt’;

6、根据查询结果插入表中
insert into tag (tagID) select distinct(tagID) from dataall;

7、按照分组，找出每组中前几条数据，并写入文件
select a.*
from 201010base11 a
where ( select count(1)
from 201010base11
where userID=a.userID and score > a.score ) <2
order by a.userID,a.score desc into outfile ‘D:\Topn.txt’;

8、计算一个表中，与另一个表的重复数据
select count(1) from topn a,(select userID,bookmarkID,tagID from 201010test1)b where a.userID=b.userID AND a.bookmarkID=b.bookmarkID AND a.tagID= b.tagID;

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