## 

1. 
2.  ,   
3. 
4. IOIO
5. 

## 

1.  
2.  
3.  

## 

1.  

2. WHERE 

3.   

4. 

5.   

6.  where 

7. InnoDB

8. ~~ NOT NULL ~~ 

   [issue758](https://github.com/Snailclimb/JavaGuide/issues/758) **default nullwhere  null ** not nullnullnull

   MySQLAnd, in case youre wondering, allowing NULL values in the index really doesnt impact performance  NULL https://juejin.im/post/5d5defc2518825591523a1db ,[MySQLIS NULLIS NOT NULL!=](https://juejin.im/post/5d5defc2518825591523a1db) 

9.  MySQL 5.7  sys  chema_unused_indexes 

10.  limit offset 

## Mysql

### 

BTree

### BTree

## MyISAMInnoDBBTree

### MyISAM

B+TreedataB+TreeKey data  data 

### InnoDB

MyISAMB+TreedatakeyInnoDBdataMyISAMkey  PSJava

## 

### 

InnoDB+

###  

(username,age) sql 

```sql
select username , age from user where username = 'Java' and age = 22
```



## 3

1. (SSDI/O
2. 1/0I/OGROUPBY
3. 
   1
   

## 

> 
>   https://juejin.im/post/5b55b842f265da0f9e589e79
>   Java3y

###  MySQL 

MySQL()

![MySQL](http://my-blog-to-use.oss-cn-beijing.aliyuncs.com/18-10-2/28559421.jpg)

![](http://my-blog-to-use.oss-cn-beijing.aliyuncs.com/18-10-2/82053134.jpg)

 - ****
 -   ****
       - 
       - ()

select * from user where indexname = 'xxx'sql

1. ****
2. ****

On


### 

()

![](http://my-blog-to-use.oss-cn-beijing.aliyuncs.com/18-10-2/5373082.jpg)

id8

![](http://my-blog-to-use.oss-cn-beijing.aliyuncs.com/18-10-2/89338047.jpg)

 **** O(logn)

B+B+

## 

> Java


### 

MySQLUsernamecity(name,city)        

```                                                                                       
select * from user where name=xx and city=xx ; 
select * from user where name=xx ; // 
select * from user where city=xx ; //             
```
 `city= xx and name xx`

ORDER BY

### 

   name,city name  

MySQL 5.7  sys  `schema_redundant_indexes`              

### Mysql

1.PRIMARY KEY

```
ALTER TABLE `table_name` ADD PRIMARY KEY ( `column` ) 
```
2.UNIQUE() 

```
ALTER TABLE `table_name` ADD UNIQUE ( `column` ) 
```

3.INDEX() 

```
ALTER TABLE `table_name` ADD INDEX index_name ( `column` )
```

4.FULLTEXT() 

```
ALTER TABLE `table_name` ADD FULLTEXT ( `column`) 
```

5.

```
ALTER TABLE `table_name` ADD INDEX index_name ( `column1`, `column2`, `column3` )
```


## 

- Java
- MySQL_3
- https://juejin.im/post/5b55b842f265da0f9e589e79