常用数据库语句（4）

分组
SELECT
count(*) sum,
book_name,
bokk_author
FROM
book
GROUP BY
bokk_author
HAVING
sum > 3;

drop truncate delete 的简单区别（待完善）

Dorp 表结构都会删掉，全部删掉
Truncate 清空表数据，删除整个数据
Delete 删除数据，删除部分职能用delete

truncate table author
drop table test
delete from author where author_name = "hhq"

Union\union all 联合查询
union和union all关键字都是将两个结果集合并为一个,两个表的列数要相同
Union 可以去重，union all 显示所有

select bokk_author from book union all select author_name from author;
select bokk_author from book union select author_name from author;

事务处理
Mysql事务处理主要有两种方法：
1、用begin，rollback,commit来实现
Begin 开始一个事务
Rollback事务回滚
Commit事务确认

begin;
delete from author;
select from author;
ROLLBACK;
SELECT from author;
COMMIT;
2、直接用set改变mysql的自动提交模式
Set autocommit=0 禁止自动提交
Set autocommit=1 开启自动提交