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MySQL之知识点(七)
2022-07-05 16:57:00 【flag:卷王!】
一、流程控制函数
流程处理函数可以根据不同的条件,执行不同的处理流程,可以在SQL语句中实现不同的条件选择。MySQL中的流程处理函数主要包括IF()、IFNULL()和CASE()函数。
IF用法:
mysql> select if(1>0,'正确','错误'),if(1<0,'正确','错误') from dual;
mysql> select if(1>0,'错误','正确'),if(1<0,'错误','正确') from dual;
二、加密与解密函数
加密与解密函数主要用于对数据库中的数据进行加密和解密处理,防止数据被他人盗取。这些函数在保证数据库安全时非常有用。
注意:在mysql8.0中,password(),encode()、decode()已经弃用。
mysql> select md5('mysql'),sha('mysql'),md5(md5('mysql')) from dual;
(注意:md5是不可逆的)
三、MySQL信息函数
MySQL中内置了一些可以查询MySQL信息函数,这些函数主要用于帮助数据库开发或运维人员更好的对数据库进行维护工作。
mysql> select version(),connection_id(),database(),schema(),user(),current_user() from dual;
四、其他函数
MySQL中有些函数无法对其进行具体分类,但是这些函数在MySQL的开发和运维过程中也是不可忽略的。
对于format来说当n小于等于0时,则只保留整数部分
mysql> select format(123.123,2),format(123.123,-2) from dual;
conv:
mysql> select conv(16,10,2),conv(8888,10,16),conv(null,10,2) from dual;
inet_aton,inet_ntoa:
mysql> select inet_aton('192.168.1.100'),inet_ntoa(3232235876) from dual;(而3232235876是由192乘以256的3次方,加上168乘以256的2次方,加上1乘以256,再加上100)
五、聚合函数
聚合函数(分组函数):是对一组数据进行汇总的函数,输入的是一组数据的集合,输出的是单个值。
1,常见的聚合函数
1)AVG / SUM
mysql> select avg(id),sum(id) from employees;
2)MAX / MIN(适用于数值类型、字符串类型、日期时间类型的字段或变量)
mysql> select max(id),min(id) from employees;
3)COUNT
(1)作用:计算指定字段在查询结构中出现的次数
mysql> select count(id) from employees;
(2)注意:计算指定字段出现的个数时,是不计算NULL值的。
(3)AVG=SUM / COUNT
mysql> select avg(id),sum(id)/count(id) from employees;
2,group by的使用
mysql> select id from employees group by id;
注意:
1,select中出现的非组函数的字段必须申明在group by中,反之,group by中申明的字段可以不出现在select中。
2,group by申明在from后、where后面、order by前面、limit前面
3,MySQL中group by中使用with rollup,可以得到新增一条平均值结果
mysql> select id from employees group by id with rollup;
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