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面试突击63:MySQL 中如何去重?
2022-07-06 09:39:00 【InfoQ】
1.创建测试数据
-- 创建测试表
drop table if exists pageview;
create table pageview(
id bigint primary key auto_increment comment '自增主键',
aid bigint not null comment '文章ID',
uid bigint not null comment '(访问)用户ID',
createtime datetime default now() comment '创建时间'
) default charset='utf8mb4';
-- 添加测试数据
insert into pageview(aid,uid) values(1,1);
insert into pageview(aid,uid) values(1,1);
insert into pageview(aid,uid) values(2,1);
insert into pageview(aid,uid) values(2,2);
2.distinct 使用
SELECT DISTINCT column_name,column_name FROM table_name;
2.1 单列去重
2.2 多列去重
2.3 聚合函数+去重
3.group by 使用
SELECT column_name,column_name FROM table_name
WHERE column_name operator value
GROUP BY column_name
3.1 单列去重
3.2 多列去重
3.3 聚合函数 + group by
4.distinct 和 group by 的区别
区别1:查询结果集不同
区别2:使用业务场景不同
区别3:性能不同
总结
参考 & 鸣谢
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