sql常见面试（2）

1.sql 删除表中重复数据保留一条

1）删除表中多余的重复记录，重复记录是根据单个字段（peopleId）来判断，只留有rowid最小的记录 

delete from people 
where   peopleName in (select peopleName    from people group by peopleName      having count(peopleName) > 1) 
and   peopleId not in (select min(peopleId) from people group by peopleName     having count(peopleName)>1) 

 

2）删除表中多余的重复记录（多个字段），只留有rowid最小的记录 

delete from vitae a 
where (a.peopleId,a.seq) in (select peopleId,seq from vitae group by peopleId,seq having count(*) > 1) 
and rowid not in (select min(rowid) from vitae group by peopleId,seq having count(*)>1) 

 

2.查询出每门课都大于80 分的学生姓名

select distinct name from table where name not in (select distinct name from table where fenshu<=80)

select name from table group by name having min(fenshu)>80

 

3.删除除了自动编号不同, 其他都相同的学生冗余信息

自动编号   学号   姓名 课程编号 课程名称 分数
1        2005001 张三 0001     数学    69
2        2005002 李四 0001      数学    89
3        2005001 张三 0001      数学    69

delete tablename where 自动编号 not in(select min( 自动编号) from tablename group by学号, 姓名, 课程编号, 课程名称, 分数)

 

4.面试题：怎么把这样一个表儿
year   month amount
1991   1     1.1
1991   2     1.2
1991   3     1.3
1991   4     1.4
1992   1     2.1
1992   2     2.2
1992   3     2.3
1992   4     2.4
查成这样一个结果
year m1   m2   m3   m4
1991 1.1 1.2 1.3 1.4
1992 2.1 2.2 2.3 2.4 

答案一、
select year, 
(select amount from   aaa m where month=1   and m.year=aaa.year) as m1,
(select amount from   aaa m where month=2   and m.year=aaa.year) as m2,
(select amount from   aaa m where month=3   and m.year=aaa.year) as m3,
(select amount from   aaa m where month=4   and m.year=aaa.year) as m4
from aaa   group by year