Mysql面试题（查询重复数据删除重复数据）

news2024/12/24 2:25:06

Create table A (id int) 注意：id列非自增，由代码产生并输入，但代码可能产生重复id

1.业务定义中，id列不允许重复，用什么方式保证重复的id不会被输入表中？

2.若已经发生数据重复，请写出SQL语句，找出重复数据，删除重复

以下面为例子：

create table b(id int);

insert into b values(1),(2),(2),(2),(2),(2),(3),(3),(2),(4);

-- 查询重复

select id,count(*) from b group by id having count(*)>1;

查询重复结果：

完全重复：

-- 全重 
create table b2 like b;

insert into b2 select distinct * from b;

truncate b;

insert into b select * from b2;

drop table b2;

show tables ;

select * from b;

局部重复并删除重复：

create table c(
    id int unsigned auto_increment primary key ,
    name varchar(50),
    age tinyint unsigned default 18
);
insert into c values (null,'李四',20),(null,'王五',20),(null,'李四',30),(null,'张三',20),(null,'张三',20),(null,'张三',20);

select * from c;
select name,count(*) from c group by name;
-- 删除姓名重复的
delete from c where id not in (select * from (select max(id) from c group by name) as t);
-- 删除姓名和年龄
delete from c where id not in(
select max(id) from (select id,concat(name,age) info from c) t1 group by info);