Mysql Explain 详解[强烈推荐]
Mysql Explain 详解
一.语法
explain < table_name >
例如: explain select * from t3 where id=3952602;
二.explain输出解释
+----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+-------------------+---------+---------+-------+------+-------+
1.id
我的理解是SQL执行的顺利的标识,SQL从大到小的执行.
例如:
mysql> explain select * from (select * from ( select * from t3 where id=3952602) a) b;
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
| 1 | PRIMARY | <derived2> | system | NULL | NULL | NULL | NULL | 1 | |
| 2 | DERIVED | <derived3> | system | NULL | NULL | NULL | NULL | 1 | |
| 3 | DERIVED | t3 | const | PRIMARY,idx_t3_id | PRIMARY | 4 | | 1 | |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
很显然这条SQL是从里向外的执行,就是从id=3 向上执行.
2. select_type
就是select类型,可以有以下几种
(1) SIMPL
相关文档：
在运行mysql administrator的时候,无法登录mysql服务器,查找错误,老是无法进行.在进入windows下的管理工具的服务中,依然无法启动,提示说是错误1067.解决方案如下:
1\进入命令提示符,输入 sc delete mysql
提示若是delete service success,证明删除服务成功;
2\运行mysql的bin目录下面的mysqlinstanceconfig,对mysql进� ......
1.通过修改MYSQL数据库中MYSQL库的USER表
就用普通的UPDATE、INSERT语句就可以
use mysql
update user set Password=password('newpassword') where User='root';
flush privileges;
（这个亲自实验过，可以执行！）
2.在命令行中使用如下命令
&nbs ......
MYSQL 获取当前日期及日期格式
获取系统日期： NOW()
格式化日期： DATE_FORMAT(date, format)
注： date：时间字段
......
查询及删除重复记录的方法
(一)
1、查找表中多余的重复记录，重复记录是根据单个字段（peopleId）来判断
select * from people
where peopleId in (select peopleId from people group by peopleId having count(peopleId) > 1)
2、删除表中多余的重复记录，重复记录是根据单个字段（peopleId）来判断，只� ......
<!--
/* Font Definitions */
@font-face
{font-family:宋体;
panose-1:2 1 6 0 3 1 1 1 1 1;
mso-font-alt:SimSun;
mso-font-charset:134;
mso-generic-font-family:auto;
mso-font-pitch:variable;
mso-font-signature:3 135135232 16 0 262145 0;}
@font-face
{font-family:"\@宋体" ......
