Mysqlsql
一. 正则表达式
二. 查询
1. show status
这次登录以来操作多少次
Show status like ‘com_insert%’
Show status like ‘com_update%’
Show status like ‘com_select%’
Show status like ‘com_delete%’
2. show status like 'innodb_rows%'
数据库历史操作以来增删改查的影响行数
3. show status like 'connections%'
Mysql,,
三. Mysql
1. ,
2. ,(,)
3. like,
Like “%l”,
Like “%l%”,
4. orunion all
Or
Union all
Select * from user where name=’a’ union all select * from user where age=’20’
5. .
:Where truncate(price)=1
:where price>1 and price<2
6. ,不加索引,数字加上引号转为字符串,加索引
Where tel=12345678901 不加索引
Where tel=’12345678901’加索引
7. ,加索引
Select * from table where age+300>500 不加索引
Select * from table where age>500 加索引
8. ,必须要包括第一列
:
Alter table test add index(a,b,c)
不适用索引
Where b=1 and c=1
Where b=1
Where c=1
使用索引
Where a=1 b=1 c=1
Where a=1 b=1
Where a=1 c=1
Where a=1
9. is null 或者 is not null,使用时不加索引
10. (!=)不会使用索引
11. Order by 列,必须包含所有的相同的索引,才会使用索引
Order by 中,不能同时包含desc,asc
:
Alter table test add index(a,b)
Alter table test add index(c)
:
Select * from test where a=1 and c=1
Select * from test where b=1
Select * from test order by a asc,d desc
:
Select * from test where a=1 and b=1
Select * from test where a=1 order by b asc
Select * from test where order by a desc,b desc
Select * from test where c=1 order by c asc
12. 索引不是越多越好
select *
(join)代替子查询
,表连接>(not)exists>(not)in
,exists其次,in最差.