mysql

目录

• 1.关系型数据库
  • 1.1常用的关系型数据库
  • 1.2专业名词
  • 1.3关系型数据库常见组件
  • 1.4SQL语句
• 2. 数据库安装与配置
  • MariaDB
• 3.数据库操作
  • 3.1 DDL操作
    • 3.1.1用户操作
      • 创建用户
      • 删除用户
    • 3.1.2数据库操作
      • 创建数据库
      • 删除数据库
    • 3.1.3表操作
      • 创建表
      • 删除表
    • 3.1.4查看show
      • show character set
      • show engines
      • show databases
      • show tables from db_name
      • desc db_name.tb_name
      • show create table db_name.tb_name
      • show table status like 'tb_name'\G
      • 查询帮助
  • 3.2 DCL操作
    • 权限类型
    • 操作对象
    • 3.2.1创建权限grant
    • 3.2.2 查看权限
      • 查看当前用户权限
      • 查看指定用户的权限
    • 3.2.3 取消授权Revoke
  • 3.3DML操作
    • 3.3.1 INSERT
    • 3.3.2 UPDATE
    • 3.3.3 DELETE
    • 3.3.4 TRUNCATE
    • 3.3.5 SELECT
• 4.连接查询
  • 4.1 内连接
  • 4.2 外连接
    • 左连接
    • 右连接
  • 4.3全连接
• 5.分组查询
  • 5.1 语法
  • 5.2 查询

1.关系型数据库

1.1常用的关系型数据库

• Msyql,MariaDB,Percona-Server
• Posrgresql
• Oracle
• MSsql

1.2专业名词

-sql：Structure Query Language，结构化查询语言

-约束：

主键约束：一个或多个字段的组合，填入的数据必须能在本表中唯一标识本行。且必须提供数据，不能为空（NOT NULL）。一个表只能存在一个

惟一键约束：一个或多个字段的组合，填入的数据必须能在本表中唯一标识本行。允许为空（NULL）一个表可以存在多个

外键约束：一个表中的某字段可填入数据取决于另一个表的主键已有的数据
检查性约束

• 索引：将表中的一个或多个字段中的数据复制一份另存，并且这些数据需要按特定次序排序存储

1.3关系型数据库常见组件

• database:数据库
• table:表，由行(row)和列(colum)组成
  -index: 索引
  -view:视图
  -user:用户
  -privilege:权限
  -procedure:存储过程
  -function:存储函数
  -trigger:触发器
  -event scheduler:事件调度器

1.4sql语句

• DDL：数据定义语言
• DML：数据操纵语言
• DCL：数据控制语言

sql语句类型	对应操作
DDL	CREATE：创建 DROP：删 ALTER：修改
DML	INSERT：向表中插入数据 DELETE：删除表中数据 PDATE：更新表中数据 SELECT：查询表中数据
DCL	GRANT：授权 REVOKE：移除授权

2. 数据库安装与配置

MariaDB

• 1.通过yum安装

[root@localhost ~]# yum -y install mariadb mariadb-server

• 2.启动MariaDB服务

##3306端口
[root@localhost ~]# systemctl start mariadb
[root@localhost ~]# systemctl status mariadb
● mariadb.service - MariaDB database server
   Loaded: loaded (/usr/lib/systemd/system/mariadb.service; disabled; vendor preset: disabled)
   Active: active (running) since 二 2020-10-20 22:46:36 CST; 6s ago
  Process: 27817 ExecStartPost=/usr/libexec/mariadb-wait-ready $MAINPID (code=exited, status=0/SUCCESS)
  Process: 27727 ExecStartPre=/usr/libexec/mariadb-prepare-db-dir %n (code=exited, status=0/SUCCESS)
[root@localhost ~]# ss -antl
State      Recv-Q Send-Q Local Address:Port               Peer Address:Port              
LISTEN     0      5      192.168.122.1:53                       *:*                  
LISTEN     0      128          *:22                       *:*                  
LISTEN     0      128    127.0.0.1:631                      *:*                  
LISTEN     0      100    127.0.0.1:25                       *:*                  
LISTEN     0      5            *:902                      *:*                  
LISTEN     0      50           *:3306                     *:* 

• 3.查看版本

##此时说明安装成功
[root@localhost ~]# MysqLadmin --version
MysqLadmin  Ver 9.0 distrib 5.5.65-MariaDB, for Linux on x86_64

• 4.运行MysqL

##刚开始默认root用户登陆，且密码为空
[root@localhost ~]# MysqL
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 2
Server version: 5.5.65-MariaDB MariaDB Server

copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]> 

##设置root用户密码
[root@localhost ~]# MysqLadmin -u root password xxxxxx
[root@localhost ~]# MysqL -u root -p
Enter password: 
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 14
Server version: 5.5.65-MariaDB MariaDB Server

copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]> 

3.数据库操作

//语法：MysqL [OPTIONS] [database]
//常用的OPTIONS：
    -uUSERNAME      //指定用户名，默认为root
    -hHOST          //指定服务器主机，默认为localhost，推荐使用ip地址
    -pPASSWORD      //指定用户的密码
    -P#             //指定数据库监听的端口，这里的#需用实际的端口号代替，如-P3307
    -V              //查看当前使用的MysqL版本
    -e          //不登录MysqL执行sql语句后退出，常用于脚本

3.1 DDL操作

• CREATE：创建 DROP：删除 ALTER：修改

3.1.1用户操作

用户帐号由两部分组成，如'USERNAME'@'HOST'，表示此USERNAME只能从此HOST上远程登录

HOST用于限制此用户可通过哪些主机远程连接MysqL程序，其值可为：IP地址或者是通配符（%和_）

• %：匹配任意长度的任意字符，常用于设置允许从任何主机登录
• _：匹配任意单个字符

创建用户

CREATE USER 'username'@'host' [IDENTIFIED BY 'password'];

##创建wisan用户
MariaDB [(none)]> create user 'wisan'@'localhost' identified by '123456';
Query OK, 0 rows affected (0.00 sec)
MariaDB [(none)]> quit
Bye

##使用wisan用户登陆
[root@localhost ~]# MysqL -uwisan -h127.0.0.1 -p
Enter password: 
Welcome to the MariaDB monitor.  Commands end with ; or \g.
Your MariaDB connection id is 15
Server version: 5.5.65-MariaDB MariaDB Server

copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]> 

删除用户

DROP USER 'username'@'host';

MariaDB [(none)]> drop user 'wisan'@'localhost';
Query OK, 0 rows affected (0.00 sec)

[root@localhost ~]# MysqL -uwisan -hlocalhost -p
Enter password: 
ERROR 1045 (28000): Access denied for user 'wisan'@'localhost' (using password: YES)

3.1.2数据库操作

创建数据库

CREATE DATABASE [IF NOT EXISTS] 'DB_NAME';

##root用户登陆
##其他用户没有给权限则操作不了
MariaDB [(none)]> create database if not exists wisan_db;
Query OK, 1 row affected (0.00 sec)

MariaDB [(none)]> show databases>;
+--------------------+
| Database           |
+--------------------+
| @R_72_4045@ion_schema |
| MysqL              |
| performance_schema |
| test               |
| wisan_db           |
+--------------------+

删除数据库

DROP DATABASE [IF EXISTS] 'DB_NAME';

MariaDB [(none)]> drop database if exists wisan_db;
Query OK, 0 rows affected (0.00 sec)

MariaDB [(none)]> show databases;
+--------------------+
| Database           |
+--------------------+
| @R_72_4045@ion_schema |
| MysqL              |
| performance_schema |
| test               |
+--------------------+
4 rows in set (0.00 sec)

3.1.3表操作

创建表

CREATE TABLE table_name (col1 datatype 修饰符,col2 datatype 修饰符) ENGINE='存储引擎类型';

##先进入数据库
MariaDB [(none)]> use wisan_db;
Database changed
MariaDB [wisan_db]> create table student(id int not null,name varchar(100) not null,age tinyint);
Query OK, 0 rows affected (0.05sec)

MariaDB [wisan_db]> show tables from wisan_db;
+--------------------+
| Tables_in_wisan_db |
+--------------------+
| student            |
+--------------------+
1 row in set (0.00 sec)

MariaDB [wisan_db]> desc wisan_db.student;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id    | int(11)      | NO   |     | NULL    |       |
| name  | varchar(100) | NO   |     | NULL    |       |
| age   | tinyint(4)   | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

删除表

DROP TABLE [ IF EXISTS ] 'table_name';

MariaDB [wisan_db]> drop table if exists student;
Query OK, 0 rows affected (0.02 sec)

MariaDB [wisan_db]> show tables ;
Empty set (0.00 sec)

3.1.4查看show

show character set

MariaDB [wisan_db]> show character set ;
+----------+-----------------------------+---------------------+--------+
| Charset  | Description                 | Default collation   | Maxlen |
+----------+-----------------------------+---------------------+--------+
| big5     | Big5 Traditional Chinese    | big5_chinese_ci     |      2 |
| dec8     | DEC West European           | dec8_swedish_ci     |      1 |
| cp850    | DOS West European           | cp850_general_ci    |      1 |

show engines

MariaDB [wisan_db]> show engines ;
+--------------------+---------+----------------------------------------------------------------------------------+--------------+------+------------+
| Engine             | Support | Comment                                                                          | Transactions | XA   | Savepoints |
+--------------------+---------+----------------------------------------------------------------------------------+--------------+------+------------+
| InnoDB             | DEFAULT | Percona-XTradB, Supports transactions, row-level locking, and foreign keys       | YES          | YES  | YES        |
| MRG_MYISAM         | YES     | Collection of identical MyISAM tables                                            | NO           | NO   | NO         |
| MyISAM             | YES     | Non-transactional engine with good performance and small data footprint          | NO           | NO   | NO         |
| BLACKHOLE          | YES     | /dev/null storage engine (anything you write to it disappears)                   | NO           | NO   | NO         |
| PERFORMANCE_SCHEMA | YES     | Performance Schema                                                               | NO           | NO   | NO         |
| CSV                | YES     | Stores tables as CSV files                                                       | NO           | NO   | NO         |
| ARCHIVE            | YES     | gzip-compresses tables for a low storage footprint                               | NO           | NO   | NO         |
| MEMORY             | YES     | Hash based, stored in memory, useful for temporary tables                        | NO           | NO   | NO         |
| FEDERATED          | YES     | Allows to access tables on other MariaDB servers, supports transactions and more | YES          | NO   | YES        |
| Aria               | YES     | Crash-safe tables with MyISAM heritage                                           | NO           | NO   | NO         |
+--------------------+---------+----------------------------------------------------------------------------------+--------------+------+------------+
10 rows in set (0.00 sec)

show databases

MariaDB [wisan_db]> show databases;
+--------------------+
| Database           |
+--------------------+
| @R_72_4045@ion_schema |
| MysqL              |
| performance_schema |
| test               |
| wisan_db           |
+--------------------+
5 rows in set (0.00 sec)

show tables from db_name

MariaDB [wisan_db]> show tables from wisan_db;
+--------------------+
| Tables_in_wisan_db |
+--------------------+
| student            |
+--------------------+
1 row in set (0.00 sec)

desc db_name.tb_name

MariaDB [wisan_db]> desc wisan_db.student;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id    | int(11)      | NO   |     | NULL    |       |
| name  | varchar(100) | NO   |     | NULL    |       |
| age   | tinyint(4)   | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.01 sec)

show create table db_name.tb_name

MariaDB [wisan_db]> show create table wisan_db.student;
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                             |
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------+
| student | CREATE TABLE `student` (
  `id` int(11) NOT NULL,
  `name` varchar(100) NOT NULL,
  `age` tinyint(4) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+---------+----------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

@H_630_404@show table status like 'tb_name'\G

MariaDB [wisan_db]> show table status like 'student'\G
*************************** 1. row ***************************
           Name: student
         Engine: InnoDB
        Version: 10
     Row_format: Compact
           Rows: 0
 Avg_row_length: 0
    Data_length: 16384
Max_data_length: 0
   Index_length: 0
      Data_free: 10485760
 Auto_increment: NULL
    Create_time: 2020-10-21 00:03:03
    Update_time: NULL
     Check_time: NULL
      Collation: latin1_swedish_ci
       Checksum: NULL
 Create_options: 
        Comment: 
1 row in set (0.00 sec)

查询帮助

• HELP keyword;

MariaDB [wisan_db]> help create table;
Name: 'CREATE TABLE'
Description:
Syntax:
CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name
    (create_deFinition,...)
    [table_options]
    [partition_options]
........................................................

3.2 DCL操作

• GRANT：授权 REVOKE：移除授权

权限类型

• priv_type

WITH GRANT OPTION：被授权的用户可将自己的权限副本转赠给其他用户

类型	意义
ALL	所有权限
SELECT	读取内容的权限
INSERT	插入内容的权限
UPDATE	更新内容的权限
DELETE	删除内容的权限

操作对象

• 表: db_name.tb_name
• 存储 函数
• 存储 过程

对象	意义
* . *	所有库的所有表
db_name.*	指定库的所有表
db_name.tb_name	指定库的指定表

3.2.1创建权限grant

GRANT priv_type,... ON [object_type] db_name.table_name TO ‘username'@'host' [IDENTIFIED BY 'password'] [WITH GRANT OPTION];

##授权wisan用户本地登陆数据库访问所有库的表
MariaDB [(none)]> grant all on *.* to 'wisan'@'localhost' identified by '123456';
Query OK, 0 rows affected (0.00 sec)

##授权wisan用户在172.16.104.132远程登陆数据库访问wisan_db库的所有表
MariaDB [(none)]> grant all on wisan_db.* to 'wisan'@'172.16.104.132' identified by '123456';
Query OK, 0 rows affected (0.00 sec)

##授权wisan用户在所有位置远程登陆数据库访问所有库的所有表
MariaDB [(none)]> grant all on *.* to 'wisan'@'%' identified by '123456';
Query OK, 0 rows affected (0.00 sec)

3.2.2 查看权限

查看当前用户权限

• show grants

MariaDB [(none)]> show grants;
+-----------------------------------------------------------------------------------------------------------------------+
| Grants for wisan@localhost                                                                                            |
+-----------------------------------------------------------------------------------------------------------------------+
| GRANT ALL PRIVILEGES ON *.* TO 'wisan'@'localhost' IDENTIFIED BY PASSWORD '*6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9' |
+-----------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

查看指定用户的权限

• show grants for user

MariaDB [(none)]> show grants for 'wisan'@'localhost';
+-----------------------------------------------------------------------------------------------------------------------+
| Grants for wisan@localhost                                                                                            |
+-----------------------------------------------------------------------------------------------------------------------+
| GRANT ALL PRIVILEGES ON *.* TO 'wisan'@'localhost' IDENTIFIED BY PASSWORD '*6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9' |
+-----------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

MariaDB [(none)]> show grants for 'wisan'@'172.16.104.132';
+-------------------------------------------------------------------------------------------------------------------+
| Grants for [email protected]                                                                                   |
+-------------------------------------------------------------------------------------------------------------------+
| GRANT USAGE ON *.* TO 'wisan'@'172.16.104.132' IDENTIFIED BY PASSWORD '*6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9' |
| GRANT ALL PRIVILEGES ON `wisan_db`.* TO 'wisan'@'172.16.104.132'                                                  |
+-------------------------------------------------------------------------------------------------------------------+
2 rows in set (0.00 sec)

3.2.3 取消授权Revoke

REVOKE priv_type,... ON db_name.table_name FROM 'username'@'host';

MariaDB [(none)]> show grants for 'wisan'@'172.16.104.132';
+-------------------------------------------------------------------------------------------------------------------+
| Grants for [email protected]                                                                                   |
+-------------------------------------------------------------------------------------------------------------------+
| GRANT USAGE ON *.* TO 'wisan'@'172.16.104.132' IDENTIFIED BY PASSWORD '*6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9' |
| GRANT ALL PRIVILEGES ON `wisan_db`.* TO 'wisan'@'172.16.104.132'                                                  |
+-------------------------------------------------------------------------------------------------------------------+
2 rows in set (0.00 sec)

MariaDB [(none)]> revoke all on wisan_db.* from 'wisan'@'172.16.104.132';
Query OK, 0 rows affected (0.00 sec)

MariaDB [(none)]> show grants for 'wisan'@'172.16.104.132';
+-------------------------------------------------------------------------------------------------------------------+
| Grants for [email protected]                                                                                   |
+-------------------------------------------------------------------------------------------------------------------+
| GRANT USAGE ON *.* TO 'wisan'@'172.16.104.132' IDENTIFIED BY PASSWORD '*6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9' |
+-------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

MariaDB [(none)]> 

注意：MysqL服务进程启动时会读取MysqL库中的所有授权表至内存中：

GRANT或REVOKE等执行权限操作会保存于表中，MysqL的服务进程会自动重读授权表，并更新至内存中

对于不能够或不能及时重读授权表的命令，可手动让MysqL的服务进程重读授权表

MysqL> FLUSH PRIVILEGES;

3.3DML操作

• 增(INSERT)、删(DELETE)、改(UPDATE)、查(SELECT)，均属针对表的数据操作

3.3.1 INSERT

INSERT [INTO] table_name [(column_name,...)] {VALUES | VALUE} (value1,...),(...),...

##插入一条数据
MariaDB [(none)]> use wisan_db;
Reading table @R_72_4045@ion for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
MariaDB [wisan_db]> insert into student(id,name,age) value(1,'tom',20);
Query OK, 1 row affected (0.02 sec)


##插入多条数据
MariaDB [wisan_db]> insert into student(id,name,age) values(2,'tom',20),(3,'null',15),(4,'jerry',null),(5,'cat',NULL);
Query OK, 4 rows affected (0.05 sec)
Records: 4  Duplicates: 0  Warnings: 0

MariaDB [wisan_db]> select * from student;
+----+-------+------+
| id | name  | age  |
+----+-------+------+
|  1 | tom   |   20 |
|  2 | tom   |   20 |
|  3 | null  |   15 |
|  4 | jerry | NULL |
|  5 | cat   | NULL |
+----+-------+------+
5 rows in set (0.00 sec)

3.3.2 UPDATE

UPDATE table_name SET column1 = new_value1[,column2 = new_value2,...] [WHERE clause] [ORDER BY 'column_name' [DESC]] [LIMIT [m,]n];

MariaDB [wisan_db]> select * from student;
+----+-------+------+
| id | name  | age  |
+----+-------+------+
|  1 | tom   |   20 |
|  2 | tom   |   20 |
|  3 | null  |   15 |
|  4 | jerry | NULL |
|  5 | cat   | NULL |
+----+-------+------+
5 rows in set (0.00 sec)
'
MariaDB [wisan_db]> update student set name = 'mouse' where name = 'null';
Query OK, 1 row affected (0.02 sec)
Rows matched: 1  Changed: 1  Warnings: 0

MariaDB [wisan_db]> update student set age = 18 where id = 4;
Query OK, 1 row affected (0.31 sec)
Rows matched: 1  Changed: 1  Warnings: 0

MariaDB [wisan_db]> select * from student;
+----+-------+------+
| id | name  | age  |
+----+-------+------+
|  1 | tom   |   20 |
|  2 | tom   |   20 |
|  3 | mouse |   15 |
|  4 | jerry |   18 |
|  5 | cat   | NULL |
+----+-------+------+
5 rows in set (0.00 sec)

3.3.3 DELETE

DELETE FROM table_name [WHERE clause] [ORDER BY 'column_name' [DESC]] [LIMIT [m,]n];

MariaDB [wisan_db]> select * from student;
+----+-------+------+
| id | name  | age  |
+----+-------+------+
|  1 | tom   |   20 |
|  2 | tom   |   20 |
|  3 | mouse |   15 |
|  4 | jerry |   18 |
|  5 | cat   | NULL |
+----+-------+------+
5 rows in set (0.00 sec)

MariaDB [wisan_db]> delete from student where id = 1;
Query OK, 1 row affected (0.02 sec)

MariaDB [wisan_db]> select * from student;
+----+-------+------+
| id | name  | age  |
+----+-------+------+
|  2 | tom   |   20 |
|  3 | mouse |   15 |
|  4 | jerry |   18 |
|  5 | cat   | NULL |
+----+-------+------+
4 rows in set (0.00 sec)

MariaDB [wisan_db]> delete from student;
Query OK, 4 rows affected (0.02 sec)

MariaDB [wisan_db]> select * from student;
Empty set (0.00 sec)

MariaDB [wisan_db]> desc wisan_db.student;
+-------+--------------+------+-----+---------+-------+
| Field | Type         | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id    | int(11)      | NO   |     | NULL    |       |
| name  | varchar(100) | NO   |     | NULL    |       |
| age   | tinyint(4)   | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.01 sec)

3.3.4 TruncATE

TruncATE table_name;

truncate删除表中所有数据，新添加的行计数值重置为初始值，且无法恢复（通过释放存储表数据所用的数据页来删除数据，并且只在事务日志中记录页的释放）。表结构、约束和索引等保持不变。对于有外键约束引用的表不能使用它删除数据，也不能用于加入了索引视图的表。

delete删除表内容时仅删除内容且每次删除一行，但会保留表结构。并在事务日志中为所删除的每行记录一项，所以可以通过回滚事务日志恢复数据

3.3.5 SELECT

• select * 表示查询所有字段
• select column as alias 表示查询column并用alias替代
• 常用操作符号:< ，>，>= ，<= ，= ，!= ，BETWEEN * AND *
• LIKE（模糊匹配），RLIKE（正则表达式匹配），IS NOT NULL，IS NULL
• 逻辑操作符号: AND OR NOT
• 排序:ORDER BY COLUMN (默认ASC升序，DESC降序)，LIMIT [n],m(略过第n个)取第m个

SELECT column1,column2,... FROM table_name [WHERE clause] [ORDER BY 'column_name' [DESC]] [LIMIT [m,]n];

4.连接查询

• 笛卡尔积，表记录的乘积
• 内连接查询时，若没有条件，或者条件为真，则返回笛卡尔积
• 创建两张表student_info,student_score

MariaDB [wisan_db]> desc wisan_db.student_info;
+-------+-------------+------+-----+---------+----------------+
| Field | Type        | Null | Key | Default | Extra          |
+-------+-------------+------+-----+---------+----------------+
| id    | int(11)     | NO   | PRI | NULL    | auto_increment |
| name  | varchar(10) | NO   |     | NULL    |                |
| age   | tinyint(4)  | YES  |     | NULL    |                |
+-------+-------------+------+-----+---------+----------------+
MariaDB [wisan_db]> select * from student_info;
+----+-------+------+
| id | name  | age  |
+----+-------+------+
|  1 | xiang |   18 |
|  2 | peng  |   19 |
|  3 | fan   |   17 |
|  4 | yi    |   20 |
|  5 | jing  | NULL |
|  6 | wisan |   19 |
|  7 | flora |   18 |
|  8 | wang  | NULL |
+----+-------+------+


MariaDB [wisan_db]> desc wisan_db.student_score;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | NO   | PRI | NULL    |       |
| name  | varchar(10) | NO   |     | NULL    |       |
| score | tinyint(4)  | YES  |     | NULL    |       |
| grade | char(1)     | NO   |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
MariaDB [wisan_db]> select * from student_score;
+----+-----------+-------+-------+
| id | name      | score | grade |
+----+-----------+-------+-------+
|  2 | peng      |    88 | A     |
|  3 | fan       |    90 | A     |
|  5 | jing      |   100 | A     |
|  6 | wisan     |  NULL | C     |
|  7 | flora     |    85 | B     |
|  8 | wang      |  NULL | C     |
| 10 | lisi      |    60 | B     |
| 11 | biyue     |    85 | B     |
| 13 | zhangshan |    70 | B     |
| 15 | qiuwu     |    70 | B     |
+----+-----------+-------+-------+

4.1 内连接

• 说明：组合两个表中的记录，返回满足条件的两个表交集部分。
• 关键字：inner join on 连接条件
• 语句：select * from a_table inner join b_table on a_table.id = b_table.id;

##没有条件时
MariaDB [wisan_db]> select * from student_info as a inner join student_score as b;
+----+-------+------+----+-----------+-------+-------+
| id | name  | age  | id | name      | score | grade |
+----+-------+------+----+-----------+-------+-------+
|  1 | xiang |   18 |  2 | peng      |    88 | A     |
|  2 | peng  |   19 |  2 | peng      |    88 | A     |
|  3 | fan   |   17 |  2 | peng      |    88 | A     |
|  4 | yi    |   20 |  2 | peng      |    88 | A     |
|  5 | jing  | NULL |  2 | peng      |    88 | A     |
|  6 | wisan |   19 |  2 | peng      |    88 | A     |
|  7 | flora |   18 |  2 | peng      |    88 | A     |
|  8 | wang  | NULL |  2 | peng      |    88 | A     |
|  1 | xiang |   18 |  3 | fan       |    90 | A     |
|  2 | peng  |   19 |  3 | fan       |    90 | A     |
|  3 | fan   |   17 |  3 | fan       |    90 | A     |
|  4 | yi    |   20 |  3 | fan       |    90 | A     |
|  5 | jing  | NULL |  3 | fan       |    90 | A     |
|  6 | wisan |   19 |  3 | fan       |    90 | A     |
|  7 | flora |   18 |  3 | fan       |    90 | A     |
....................................................
##一共有8*10条记录

##有条件时（a.id=b.id），此时输出表的交集
MariaDB [wisan_db]> select * from student_info as a inner join student_score as b on a.id = b.id;
+----+-------+------+----+-------+-------+-------+
| id | name  | age  | id | name  | score | grade |
+----+-------+------+----+-------+-------+-------+
|  2 | peng  |   19 |  2 | peng  |    88 | A     |
|  3 | fan   |   17 |  3 | fan   |    90 | A     |
|  5 | jing  | NULL |  5 | jing  |   100 | A     |
|  6 | wisan |   19 |  6 | wisan |  NULL | C     |
|  7 | flora |   18 |  7 | flora |    85 | B     |
|  8 | wang  | NULL |  8 | wang  |  NULL | C     |
+----+-------+------+----+-------+-------+-------+

4.2 外连接

左连接

• 说明：返回左表(即关键词左边的表)所有记录和右表满足条件的记录(不足的NULL代替)
• 关键字：left join on 连接条件
• 语句：select * from a_table left join b_table on a_table.id = b_table.id;

##
MariaDB [wisan_db]> select * from student_info as a left join student_score as b on a.id = b.id;
+----+-------+------+------+-------+-------+-------+
| id | name  | age  | id   | name  | score | grade |
+----+-------+------+------+-------+-------+-------+
|  1 | xiang |   18 | NULL | NULL  |  NULL | NULL  |
|  2 | peng  |   19 |    2 | peng  |    88 | A     |
|  3 | fan   |   17 |    3 | fan   |    90 | A     |
|  4 | yi    |   20 | NULL | NULL  |  NULL | NULL  |
|  5 | jing  | NULL |    5 | jing  |   100 | A     |
|  6 | wisan |   19 |    6 | wisan |  NULL | C     |
|  7 | flora |   18 |    7 | flora |    85 | B     |
|  8 | wang  | NULL |    8 | wang  |  NULL | C     |
+----+-------+------+------+-------+-------+-------+
8 rows in set (0.001 sec)

右连接

• 说明：返回右表(即关键词右边的表)所有记录和左表满足条件的记录(不足的NULL代替)
• 关键字：right join on 连接条件
• 语句：select * from a_table right join b_table on a_table.id = b_table.id;

MariaDB [wisan_db]> select * from student_info as a right join student_score as b on a.id = b.id;
+------+-------+------+----+-----------+-------+-------+
| id   | name  | age  | id | name      | score | grade |
+------+-------+------+----+-----------+-------+-------+
|    2 | peng  |   19 |  2 | peng      |    88 | A     |
|    3 | fan   |   17 |  3 | fan       |    90 | A     |
|    5 | jing  | NULL |  5 | jing      |   100 | A     |
|    6 | wisan |   19 |  6 | wisan     |  NULL | C     |
|    7 | flora |   18 |  7 | flora     |    85 | B     |
|    8 | wang  | NULL |  8 | wang      |  NULL | C     |
| NULL | NULL  | NULL | 10 | lisi      |    60 | B     |
| NULL | NULL  | NULL | 11 | biyue     |    85 | B     |
| NULL | NULL  | NULL | 13 | zhangshan |    70 | B     |
| NULL | NULL  | NULL | 15 | qiuwu     |    70 | B     |
+------+-------+------+----+-----------+-------+-------+

4.3全连接

• 关键字：union | union all
• 语法: (查询语句) union|union all (查询语句)
• 注意：

1.查询的COLUMN数(列数)必须相等
2.没有要求时，合并的表列名以第一个查询语句为准
3.union会合并完全相同的记录，但是比较耗时，union all则不会，通常使用union all
4.在单个查询的语句中即便排序，也不会有排序效果，但是可以对最终结果排序

##union查询name，age字段,两条记录（wang null）合并为一条
MariaDB [wisan_db]> (select name,age from student_info) union (select name,score from student_score);
+-----------+------+
| name      | age  |
+-----------+------+
| xiang     |   18 |
| peng      |   19 |
| fan       |   17 |
| yi        |   20 |
| jing      | NULL |
| wisan     |   19 |
| flora     |   18 |
| wang      | NULL |   ####
| peng      |   88 |
| fan       |   90 |
| jing      |  100 |
| wisan     | NULL |
| flora     |   85 |
| lisi      |   60 |
| biyue     |   85 |
| zhangshan |   70 |
| qiuwu     |   70 |
+-----------+------+


## union all查询name,score字段
MariaDB [wisan_db]> (select name,age from student_info) union all (select name,score from student_score);
+-----------+------+
| name      | age  |
+-----------+------+
| xiang     |   18 |
| peng      |   19 |
| fan       |   17 |
| yi        |   20 |
| jing      | NULL |
| wisan     |   19 |
| flora     |   18 |
| wang      | NULL |    ######
| peng      |   88 |
| fan       |   90 |
| jing      |  100 |
| wisan     | NULL |
| flora     |   85 |
| wang      | NULL |    ######
| lisi      |   60 |
| biyue     |   85 |
| zhangshan |   70 |
| qiuwu     |   70 |
+-----------+------+
18 rows in set (0.001 sec)


##在第二个查询语句排序，结果并没有排序
MariaDB [wisan_db]> (select name,age from student_info) union all (select name,score from student_score order by score);
+-----------+------+
| name      | age  |
+-----------+------+
| xiang     |   18 |
| peng      |   19 |
| fan       |   17 |
| yi        |   20 |
| jing      | NULL |
| wisan     |   19 |
| flora     |   18 |
| wang      | NULL |
| peng      |   88 |
| fan       |   90 |
| jing      |  100 |
| wisan     | NULL |
| flora     |   85 |
| wang      | NULL |
| lisi      |   60 |
| biyue     |   85 |
| zhangshan |   70 |
| qiuwu     |   70 |
+-----------+------+

##对整个结果排序
MariaDB [wisan_db]> (select name,age from student_info) union all (select name,score from student_score order by score) order by age;
+-----------+------+
| name      | age  |
+-----------+------+
| jing      | NULL |
| wang      | NULL |
| wisan     | NULL |
| wang      | NULL |
| fan       |   17 |
| flora     |   18 |
| xiang     |   18 |
| peng      |   19 |
| wisan     |   19 |
| yi        |   20 |
| lisi      |   60 |
| qiuwu     |   70 |
| zhangshan |   70 |
| flora     |   85 |
| biyue     |   85 |
| peng      |   88 |
| fan       |   90 |
| jing      |  100 |
+-----------+------+

5.分组查询

5.1 语法

SELECT column, group_function,... FROM table [WHERE condition] GROUP BY group_by_expression [HAVING group_condition];

• 说明

group_function：聚合函数。
group_by_expression：分组表达式，多个之间用逗号隔开。
group_condition：分组之后对数据进行过滤。
分组中，select后面只能有两种类型的列：

• group_function：

函数名	作用
max	查询指定列的最大值
min	查询指定列的最小值
count	统计查询结果的行数
sum	求和，返回指定列的总和
avg	求平均值，返回指定列数据的平均值

5.2 查询

• 建表

MariaDB [wisan_db]> select * from student_info;
+----+-------+------+
| id | name  | age  |
+----+-------+------+
|  1 | xiang |   18 |
|  2 | peng  |   19 |
|  3 | fan   |   17 |
|  4 | yi    |   20 |
|  5 | jing  | NULL |
|  6 | wisan |   19 |
|  7 | flora |   18 |
|  8 | wang  | NULL |
+----+-------+------+
8 rows in set (0.001 sec)

MariaDB [wisan_db]> select * from student_score;
+----+-----------+-------+-------+
| id | name      | score | grade |
+----+-----------+-------+-------+
|  2 | peng      |    88 | A     |
|  3 | fan       |    90 | A     |
|  5 | jing      |   100 | A     |
|  6 | wisan     |  NULL | C     |
|  7 | flora     |    85 | B     |
|  8 | wang      |  NULL | C     |
| 10 | lisi      |    60 | B     |
| 11 | biyue     |    85 | B     |
| 13 | zhangshan |    70 | B     |
| 15 | qiuwu     |    70 | B     |
+----+-----------+-------+-------+

• 单独查询

##每个组只取第一个
MariaDB [wisan_db]> select * from student_score group by grade;
+----+-------+-------+-------+
| id | name  | score | grade |
+----+-------+-------+-------+
|  2 | peng  |    88 | A     |
|  7 | flora |    85 | B     |
|  6 | wisan |  NULL | C     |
+----+-------+-------+-------+

• group_concat()函数输出里面的所有成员

MariaDB [wisan_db]> select group_concat(name),group_concat(score),grade from student_score group by grade;
+----------------------------------+---------------------+-------+
| group_concat(name)               | group_concat(score) | grade |
+----------------------------------+---------------------+-------+
| peng,fan,jing                    | 88,90,100           | A     |
| zhangshan,biyue,lisi,qiuwu,flora | 70,85,60,70,85      | B     |
| wisan,wang                       | NULL                | C     |
+----------------------------------+---------------------+-------+

• count()函数统计里面的成员数量

MariaDB [wisan_db]> select count(name),grade from student_score group by grade;
+-------------+-------+
| count(name) | grade |
+-------------+-------+
|           3 | A     |
|           5 | B     |
|           2 | C     |
+-------------+-------+

• HAVING，对分组后的结果筛选,分组人数>=3

MariaDB [wisan_db]> select count(name),grade from student_score group by grade having count(name)>=3;
+-------------+-------+
| count(name) | grade |
+-------------+-------+
|           3 | A     |
|           5 | B     |
+-------------+-------+

• with roollup在最后进行统计

MariaDB [wisan_db]> select count(name),grade from student_score group by grade with rollup;
+-------------+-------+
| count(name) | grade |
+-------------+-------+
|           3 | A     |
|           5 | B     |
|           2 | C     |
|          10 | NULL  |
+-------------+-------+