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mysql的基本操作

仲秋花似锦 2022-01-04 阅读 83
mysql

1.mysql中的注释

        # #号注释
        -- 双横杠注释

2.mysql的查询操作

(1).查询stu_info数据库中的全部信息

        select * from stu_info;

(2).查询stu_info数据库中属性为name的信息

        select name from stu_info;

3.mysql的添加操作

(1).向stu_info数据库的stu_name和sex属性添加信息

        insert into stu_info(stu_name,sex) values('张三',1);
        insert into stu_info(stu_name,sex) values('李四',0);
        insert into stu_info(stu_name,sex) values('王五',0);

(2).向cla_info数据库中添加信息

        insert into cla_info set cla_name = '舞蹈班';    
        insert into cla_info set cla_name = '舞蹈班',cla_teacher='罗翔';

(3).批量插入
        insert into stu_info(stu_name,sex) select n_name,n_sex from new;

4.mysql的更新操作

(1).更新stu_info数据库中id=2(id为主键,具有唯一性)的数据的grade=89

        update stu_info set grade = 89 where id=2;

5.mysql的删除操作

(1).删除stu_info数据库中id=3的数据

        delete from stu_info where id=3

(2).慎重使用(将stu_info数据库中的信息全部删除)

        delete from stu_info

6.mysql常用聚合函数

(1).求总行数
        select count(*) from stu_info;
    
(2).求平均值
        select avg(grade) as avg_grade from stu_info;
    
(3).求最大值,并将其命名为max_grade

        select max(grade) as max_grade from stu_info;


(4).求最小值    

        select min(grade) as min_grade from stu_info;


(5).求和    

        select sum(grade) from stu_info;

(6).保留小数点后几位,四舍五入
        select round(3.141592,4);

 
(7).查询当前数据库版本
        select version();
    
(8).当前数据库系统所在服务器时间
        select now();
    
(9).日期格式化
        select DATE_FORMAT(now(),'%Y-%m-%d %H:%i:%s');
    
(10).日期增加或减少
        select  date_add(now(),interval 1 day);
    
        select  date_add(now(),interval -1 day);
        
         select  date_add(now(),interval 1 hour);
    
(11).拼接字符串
        select concat(stu_name,grade) from stu_info;
    
(12).去掉重复关键字
        select distinct stu_name from stu_info;
    
(13).在distinct后面有多个字段,会对多个字段的组合进行去重    
        select distinct id,stu_name from stu_info;

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