mysql进阶

mysql进阶

1. 二进制格式mysql安装

下载二进制格式的mysql软件包
[root@localhost src]# wget https://downloads.mysql.com/archives/get/p/23/file/mysql-5.7.38-linux-glibc2.12-x86_64.tar.gz
--2022-07-27 16:01:49--  https://downloads.mysql.com/archives/get/p/23/file/mysql-5.7.38-linux-glibc2.12-x86_64.tar.gz
Resolving downloads.mysql.com (downloads.mysql.com)... 104.102.102.89, 2600:1406:c800:593::2e31, 2600:1406:c800:5ab::2e31
Connecting to downloads.mysql.com (downloads.mysql.com)|104.102.102.89|:443... connected.
HTTP request sent, awaiting response... 302 Moved Temporarily
Location: https://cdn.mysql.com/archives/mysql-5.7/mysql-5.7.38-linux-glibc2.12-x86_64.tar.gz [following]
--2022-07-27 16:01:50--  https://cdn.mysql.com/archives/mysql-5.7/mysql-5.7.38-linux-glibc2.12-x86_64.tar.gz
Resolving cdn.mysql.com (cdn.mysql.com)... 23.33.16.230
Connecting to cdn.mysql.com (cdn.mysql.com)|23.33.16.230|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 674830866 (644M) [application/x-tar-gz]
Saving to: 'mysql-5.7.38-linux-glibc2.12-x86_64.tar.gz.1'

mysql-5.7.38-linux-g 100%[=====================>] 643.57M  3.01MB/s    in 3m 8s   

2022-07-27 16:04:59 (3.43 MB/s) - 'mysql-5.7.38-linux-glibc2.12-x86_64.tar.gz.1' saved [674830866/674830866]

[root@localhost src]# 
//创建用户
[root@localhost src]# useradd -M -r -s /sbin/nologin mysql 
[root@localhost src]# id mysql
uid=995(mysql) gid=992(mysql) groups=992(mysql)
[root@localhost src]# 
//解压软件至/usr/local/
[root@localhost src]# tar xf mysql-5.7.38-linux-glibc2.12-x86_64.tar.gz -C /usr/local
[root@localhost local]# ll
total 0
drwxr-xr-x. 2 root root   6 Jun 22  2021 bin
drwxr-xr-x. 2 root root   6 Jun 22  2021 etc
drwxr-xr-x. 2 root root   6 Jun 22  2021 games
drwxr-xr-x. 2 root root   6 Jun 22  2021 include
drwxr-xr-x. 2 root root   6 Jun 22  2021 lib
drwxr-xr-x. 3 root root  17 Jun 27 21:48 lib64
drwxr-xr-x. 2 root root   6 Jun 22  2021 libexec
drwxr-xr-x. 9 root root 129 Jul 27 16:19 mysql-5.7.38-linux-glibc2.12-x86_64
drwxr-xr-x. 2 root root   6 Jun 22  2021 sbin
drwxr-xr-x. 5 root root  49 Jun 27 21:48 share
drwxr-xr-x. 2 root root   6 Jun 22  2021 src
[root@localhost local]# ln -sv mysql-5.7.38-linux-glibc2.12-x86_64/ mysql
'mysql' -> 'mysql-5.7.38-linux-glibc2.12-x86_64/'
[root@localhost local]# 
//修改目录/usr/local/mysql的属主属组
[root@localhost local]# chown -R mysql.mysql mysql*
[root@localhost local]# ll
total 0
drwxr-xr-x. 2 root  root    6 Jun 22  2021 bin
drwxr-xr-x. 2 root  root    6 Jun 22  2021 etc
drwxr-xr-x. 2 root  root    6 Jun 22  2021 games
drwxr-xr-x. 2 root  root    6 Jun 22  2021 include
drwxr-xr-x. 2 root  root    6 Jun 22  2021 lib
drwxr-xr-x. 3 root  root   17 Jun 27 21:48 lib64
drwxr-xr-x. 2 root  root    6 Jun 22  2021 libexec
lrwxrwxrwx. 1 mysql mysql  36 Jul 27 16:22 mysql -> mysql-5.7.38-linux-glibc2.12-x86_64/
drwxr-xr-x. 9 mysql mysql 129 Jul 27 16:19 mysql-5.7.38-linux-glibc2.12-x86_64
drwxr-xr-x. 2 root  root    6 Jun 22  2021 sbin
drwxr-xr-x. 5 root  root   49 Jun 27 21:48 share
drwxr-xr-x. 2 root  root    6 Jun 22  2021 src
[root@localhost local]# 
//添加环境变量
[root@localhost local]# cd /usr/local/mysql
[root@localhost mysql]# cd bin
[root@localhost bin]# pwd
/usr/local/mysql/bin
[root@localhost bin]# echo 'export PATH=$PATH:/usr/local/mysql/bin' > /etc/profile.d/mysql.sh
[root@localhost bin]# source /etc/profile.d/mysql.sh
[root@localhost bin]# which mysql
/usr/local/mysql/bin/mysql
[root@localhost bin]# 

[root@localhost mysql]# ln -sv /usr/local/mysql/include/ /usr/include/mysql
'/usr/include/mysql' -> '/usr/local/mysql/include/'
[root@localhost mysql]# vi /etc/ld.so.conf.d/mysql.conf
/usr/local/mysql/lib/
[root@localhost mysql]# vi /etc/man_db.conf 
MANDATORY_MANPATH                       /usr/local/mysql/man
//建立数据存放目录
[root@localhost ~]# mkdir /opt/data
[root@localhost ~]# chown -R mysql.mysql /opt/data
[root@localhost ~]# 
//初始化数据库
[root@localhost ~]# mysqld --initialize --user mysql --datadir /opt/data/
2022-07-27T08:49:14.851793Z 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details).
2022-07-27T08:49:15.179367Z 0 [Warning] InnoDB: New log files created, LSN=45790
2022-07-27T08:49:15.289588Z 0 [Warning] InnoDB: Creating foreign key constraint system tables.
2022-07-27T08:49:15.307614Z 0 [Warning] No existing UUID has been found, so we assume that this is the first time that this server has been started. Generating a new UUID: fe64a754-0d88-11ed-82d5-000c29071947.
2022-07-27T08:49:15.308385Z 0 [Warning] Gtid table is not ready to be used. Table 'mysql.gtid_executed' cannot be opened.
2022-07-27T08:49:15.616217Z 0 [Warning] A deprecated TLS version TLSv1 is enabled. Please use TLSv1.2 or higher.
2022-07-27T08:49:15.616260Z 0 [Warning] A deprecated TLS version TLSv1.1 is enabled. Please use TLSv1.2 or higher.
2022-07-27T08:49:15.616693Z 0 [Warning] CA certificate ca.pem is self signed.
2022-07-27T08:49:15.726864Z 1 [Note] A temporary password is generated for root@localhost: Rqskg9Xe=,JF
[root@localhost ~]# 
//生成配置文件
[root@localhost ~]# vi /etc/my.cnf
[root@localhost ~]# cat /etc/my.cnf
[mysqld]
basedir = /usr/local/mysql
datadir = /opt/data
socket = /tmp/mysql.sock
port = 3306
pid-file = /opt/data/mysql.pid
user = mysql
skip-name-resolve
sql-mode = STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
[root@localhost ~]# 
//配置服务启动脚本
[root@localhost ~]# cp /usr/lib/systemd/system/sshd.service .  //复制一个sshd的模板来编辑
[root@localhost ~]# ls
anaconda-ks.cfg  sshd.service
[root@localhost ~]# mv sshd.service mysqld.service
[root@localhost ~]# ls
anaconda-ks.cfg  mysqld.service
[root@localhost ~]# vim mysqld.service 
[root@localhost ~]# cat mysqld.service   //将模板编辑成下面内容
[Unit]
Description=mysql server daemon
After=network.target sshd-keygen.target

[Service]
Type=forking
ExecStart=/usr/local/mysql/support-files/mysql.server start
ExecStop=/usr/local/mysql/support-files/mysql.server stop
ExecReload=/bin/kill -HUP $MAINPID

[Install]
WantedBy=multi-user.target
[root@localhost ~]# mv mysqld.service /usr/lib/systemd/system/
[root@localhost ~]# ls
anaconda-ks.cfg
[root@localhost ~]# systemctl daemon-reload
[root@localhost ~]# systemctl start mysqld
[root@localhost ~]# systemctl stop firewalld
[root@localhost ~]# systemctl disable firewalld
Removed /etc/systemd/system/multi-user.target.wants/firewalld.service.
Removed /etc/systemd/system/dbus-org.fedoraproject.FirewallD1.service.
[root@localhost ~]# vim /etc/selinux/config 
[root@localhost ~]# cat /etc/selinux/config 

# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#     enforcing - SELinux security policy is enforced.
#     permissive - SELinux prints warnings instead of enforcing.
#     disabled - No SELinux policy is loaded.
SELINUX=disabled  //更改这里
# SELINUXTYPE= can take one of these three values:
#     targeted - Targeted processes are protected,
#     minimum - Modification of targeted policy. Only selected processes are protected. 
#     mls - Multi Level Security protection.
SELINUXTYPE=targeted
[root@localhost ~]# setenforce 0
[root@localhost ~]# systemctl start mysqld
[root@localhost ~]# ss -antl
State    Recv-Q   Send-Q     Local Address:Port     Peer Address:Port  Process   
LISTEN   0        128              0.0.0.0:22            0.0.0.0:*               
LISTEN   0        128                 [::]:22               [::]:*               
LISTEN   0        80                     *:3306                *:*  
//启动mysql

[root@localhost ~]# systemctl start mysqld
[root@localhost ~]# ss -anlt
State    Recv-Q   Send-Q     Local Address:Port      Peer Address:Port   Process   
LISTEN   0        128              0.0.0.0:22             0.0.0.0:*                
LISTEN   0        128                 [::]:22                [::]:*                
LISTEN   0        80                     *:3306                 *:*                
[root@localhost ~]# 
//修改密码
//使用临时密码登录
[root@localhost ~]# mysql -uroot -p'Rqskg9Xe=,JF'
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.7.38

Copyright (c) 2000, 2022, Oracle and/or its affiliates.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

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

mysql> 
mysql> set password = password('123456');
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> 

2. mysql配置文件

mysql`的配置文件为`/etc/my.cnf

配置文件查找次序：若在多个配置文件中均有设定，则最后找到的最终生效

/etc/my.cnf --> /etc/mysql/my.cnf --> --default-extra-file=/PATH/TO/CONF_FILE --> ~/.my.cnf

mysql常用配置文件参数：

参数	说明
port = 3306	设置监听端口
socket = /tmp/mysql.sock	指定套接字文件位置
basedir = /usr/local/mysql	指定MySQL的安装路径
datadir = /data/mysql	指定MySQL的数据存放路径
pid-file = /data/mysql/mysql.pid	指定进程ID文件存放路径
user = mysql	指定MySQL以什么用户的身份提供服务
skip-name-resolve	禁止MySQL对外部连接进行DNS解析 使用这一选项可以消除MySQL进行DNS解析的时间。 若开启该选项，则所有远程主机连接授权都要使用IP地址方 式否则MySQL将无法正常处理连接请求

数据库密码破解

确认数据库可以正常登录

[root@localhost ~]# vi /etc/my.cnf 
[root@localhost ~]# cat /etc/my.cnf 
[mysqld]
basedir = /usr/local/mysql
datadir = /opt/data
socket = /tmp/mysql.sock
port = 3306
pid-file = /opt/data/mysql.pid
user = mysql
skip-name-resolve
skip-grant-tables   //添加此行，跳过名称解析
sql-mode = STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
[root@localhost ~]# 
重启mysql服务
[root@localhost ~]# systemctl restart mysqld
[root@localhost ~]# mysql -uroot 
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.7.38 MySQL Community Server (GPL)

Copyright (c) 2000, 2022, Oracle and/or its affiliates.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

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

mysql> 
进入数据库修改登录密码
[root@localhost ~]# mysql -uroot 
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.7.38 MySQL Community Server (GPL)

Copyright (c) 2000, 2022, Oracle and/or its affiliates.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

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

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| sys                |
+--------------------+
4 rows in set (0.00 sec)

mysql> use mysql;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> update  user set authentication_string = password('654321') where User = 'root' and Host = 'localhost';
Query OK, 1 row affected, 1 warning (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 1

mysql> exit
Bye
[root@localhost ~]# 
使用新密码登录mysql测试
[root@localhost ~]# vi /etc/my.cnf 
[mysqld]
basedir = /usr/local/mysql
datadir = /opt/data
socket = /tmp/mysql.sock
port = 3306
pid-file = /opt/data/mysql.pid
user = mysql
skip-name-resolve
skip-grant-tables   //删除此条
sql-mode = STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
[root@localhost ~]# systemctl restart mysqld
[root@localhost ~]# mysql -uroot -p'654321'
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 2
Server version: 5.7.38 MySQL Community Server (GPL)

Copyright (c) 2000, 2022, Oracle and/or its affiliates.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

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

mysql> 

3.多表联合查询

3.1什么是多表联合查询

前面所讲的查询语句都是针对一个表的，但是在关系型数据库中，表与表之间是有联系的，所以在实际应用中，经常使用多表查询。多表查询就是同时查询两个或两个以上的表。
在 MySQL 中，多表查询主要有交叉连接、内连接、外连接、分组查询与子查询等5种。

3.2交叉连接（CROSS JOIN）

3.2.1笛卡尔积

交叉连接(CROSS JOIN)：有两种，显式的和隐式的2种，一般用来返回连接表的笛卡尔积。
笛卡尔积（Cartesian product）是指两个集合 X 和 Y 的乘积。
例如，有 A 和 B 两个集合，它们的值如下：

A = {1,2}
B = {3,4,5}

集合 A×B 和 B×A 的结果集分别表示为：

A×B={(1,3), (1,4), (1,5), (2,3), (2,4), (2,5) };
B×A={(3,1), (3,2), (4,1), (4,2), (5,1), (5,2) };

以上 A×B 和 B×A 的结果就叫做两个集合的笛卡尔积。

并且，从以上结果我们可以看出：

• 两个集合相乘，不满足交换率，即 A×B≠B×A。
• A 集合和 B 集合的笛卡尔积是 A 集合的元素个数 × B 集合的元素个数。

多表查询遵循的算法就是以上提到的笛卡尔积，表与表之间的连接可以看成是在做乘法运算。在实际应用中，应避免使用笛卡尔积，因为笛卡尔积中容易存在大量的不合理数据，简单来说就是容易导致查询结果重复、混乱。

3.2.2 交叉连接

交叉连接的语法格式如下：

SELECT <字段名> FROM <表1> CROSS JOIN <表2> [WHERE子句];

或

SELECT <字段名> FROM <表1>, <表2> [WHERE子句];

语法说明如下：

• 字段名：需要查询的字段名称。
• <表1><表2>：需要交叉连接的表名。
• WHERE 子句：用来设置交叉连接的查询条件。

注意：多个表交叉连接时，在 FROM 后连续使用 CROSS JOIN 或,即可。以上两种语法的返回结果是相同的，但是第一种语法才是官方建议的标准写法。

当连接的表之间没有关系时，我们会省略掉 WHERE 子句，这时返回结果就是两个表的笛卡尔积，返回结果数量就是两个表的数据行相乘。需要注意的是，如果每个表有 1000 行，那么返回结果的数量就有 1000×1000 = 1000000 行，数据量是非常巨大的。

创建数据库
mysql> create database runtime;
Query OK, 1 row affected (0.00 sec)

//进入数据库
mysql> use runtime;
Database changed

//创建表tb_students_info
mysql> create table tb_students_info(id int not null primary key auto_increment,name varchar(50),age tinyint,sex varchar(4),height float,course_id int);
Query OK, 0 rows affected (0.00 sec)

//给表tb_students_info增加内容
mysql> insert tb_students_info(name,age,sex,height,course_id) values('Dany',25,'M',160,1),('Green',23,'M',158,2),('Henry',23,'W',185,1),('Jane',22,'M',162,3),('Jim',24,'W',175,2),('John',21,'W',172,4),('Lily',22,'M',165,4),('Susan',23,'M',170,5),('Thomas',22,'W',178,5),('Tom',23,'W',165,5);
Query OK, 10 rows affected (0.00 sec)
Records: 10  Duplicates: 0  Warnings: 0

//创建表tb_course
mysql> create table tb_course(id int not null primary key auto_increment,course_name varchar(50));
Query OK, 0 rows affected (0.00 sec)

//给表tb_course增加内容
mysql> insert tb_course(course_name) values('Java'),('MySQL'),('Python'),('Go'),('C++');
Query OK, 5 rows affected (0.00 sec)
Records: 5  Duplicates: 0  Warnings: 0

1.查询 tb_students_info 表中的数据，SQL 语句和运行结果如下：

mysql> select * from tb_students_info;
+----+--------+------+------+--------+-----------+
| id | name   | age  | sex  | height | course_id |
+----+--------+------+------+--------+-----------+
|  1 | Dany   |   25 | M    |    160 |         1 |
|  2 | Green  |   23 | M    |    158 |         2 |
|  3 | Henry  |   23 | W    |    185 |         1 |
|  4 | Jane   |   22 | M    |    162 |         3 |
|  5 | Jim    |   24 | W    |    175 |         2 |
|  6 | John   |   21 | W    |    172 |         4 |
|  7 | Lily   |   22 | M    |    165 |         4 |
|  8 | Susan  |   23 | M    |    170 |         5 |
|  9 | Thomas |   22 | W    |    178 |         5 |
| 10 | Tom    |   23 | W    |    165 |         5 |
+----+--------+------+------+--------+-----------+
10 rows in set (0.00 sec)

2.查询 tb_course 表中的数据，SQL 语句和运行结果如下：

mysql> select * from tb_course;
+----+-------------+
| id | course_name |
+----+-------------+
|  1 | Java        |
|  2 | MySQL       |
|  3 | Python      |
|  4 | Go          |
|  5 | C++         |
+----+-------------+
5 rows in set (0.00 sec)

3.使用 CROSS JOIN 查询出两张表中的笛卡尔积，SQL 语句和运行结果如下：

mysql> select * from tb_course cross join tb_students_info;
+----+-------------+----+--------+------+------+--------+-----------+
| id | course_name | id | name   | age  | sex  | height | course_id |
+----+-------------+----+--------+------+------+--------+-----------+
|  1 | Java        |  1 | Dany   |   25 | M    |    160 |         1 |
|  2 | MySQL       |  1 | Dany   |   25 | M    |    160 |         1 |
|  3 | Python      |  1 | Dany   |   25 | M    |    160 |         1 |
|  4 | Go          |  1 | Dany   |   25 | M    |    160 |         1 |
|  5 | C++         |  1 | Dany   |   25 | M    |    160 |         1 |
|  1 | Java        |  2 | Green  |   23 | M    |    158 |         2 |
|  2 | MySQL       |  2 | Green  |   23 | M    |    158 |         2 |
|  3 | Python      |  2 | Green  |   23 | M    |    158 |         2 |
|  4 | Go          |  2 | Green  |   23 | M    |    158 |         2 |
|  5 | C++         |  2 | Green  |   23 | M    |    158 |         2 |
|  1 | Java        |  3 | Henry  |   23 | W    |    185 |         1 |
|  2 | MySQL       |  3 | Henry  |   23 | W    |    185 |         1 |
|  3 | Python      |  3 | Henry  |   23 | W    |    185 |         1 |
|  4 | Go          |  3 | Henry  |   23 | W    |    185 |         1 |
|  5 | C++         |  3 | Henry  |   23 | W    |    185 |         1 |
|  1 | Java        |  4 | Jane   |   22 | M    |    162 |         3 |
|  2 | MySQL       |  4 | Jane   |   22 | M    |    162 |         3 |
|  3 | Python      |  4 | Jane   |   22 | M    |    162 |         3 |
|  4 | Go          |  4 | Jane   |   22 | M    |    162 |         3 |
|  5 | C++         |  4 | Jane   |   22 | M    |    162 |         3 |
|  1 | Java        |  5 | Jim    |   24 | W    |    175 |         2 |
|  2 | MySQL       |  5 | Jim    |   24 | W    |    175 |         2 |
|  3 | Python      |  5 | Jim    |   24 | W    |    175 |         2 |
|  4 | Go          |  5 | Jim    |   24 | W    |    175 |         2 |
|  5 | C++         |  5 | Jim    |   24 | W    |    175 |         2 |
|  1 | Java        |  6 | John   |   21 | W    |    172 |         4 |
|  2 | MySQL       |  6 | John   |   21 | W    |    172 |         4 |
|  3 | Python      |  6 | John   |   21 | W    |    172 |         4 |
|  4 | Go          |  6 | John   |   21 | W    |    172 |         4 |
|  5 | C++         |  6 | John   |   21 | W    |    172 |         4 |
|  1 | Java        |  7 | Lily   |   22 | M    |    165 |         4 |
|  2 | MySQL       |  7 | Lily   |   22 | M    |    165 |         4 |
|  3 | Python      |  7 | Lily   |   22 | M    |    165 |         4 |
|  4 | Go          |  7 | Lily   |   22 | M    |    165 |         4 |
|  5 | C++         |  7 | Lily   |   22 | M    |    165 |         4 |
|  1 | Java        |  8 | Susan  |   23 | M    |    170 |         5 |
|  2 | MySQL       |  8 | Susan  |   23 | M    |    170 |         5 |
|  3 | Python      |  8 | Susan  |   23 | M    |    170 |         5 |
|  4 | Go          |  8 | Susan  |   23 | M    |    170 |         5 |
|  5 | C++         |  8 | Susan  |   23 | M    |    170 |         5 |
|  1 | Java        |  9 | Thomas |   22 | W    |    178 |         5 |
|  2 | MySQL       |  9 | Thomas |   22 | W    |    178 |         5 |
|  3 | Python      |  9 | Thomas |   22 | W    |    178 |         5 |
|  4 | Go          |  9 | Thomas |   22 | W    |    178 |         5 |
|  5 | C++         |  9 | Thomas |   22 | W    |    178 |         5 |
|  1 | Java        | 10 | Tom    |   23 | W    |    165 |         5 |
|  2 | MySQL       | 10 | Tom    |   23 | W    |    165 |         5 |
|  3 | Python      | 10 | Tom    |   23 | W    |    165 |         5 |
|  4 | Go          | 10 | Tom    |   23 | W    |    165 |         5 |
|  5 | C++         | 10 | Tom    |   23 | W    |    165 |         5 |
+----+-------------+----+--------+------+------+--------+-----------+
50 rows in set (0.00 sec)

由运行结果可以看出，tb_course 和 tb_students_info 表交叉连接查询后，返回了 50 条记录。可以想象，当表中的数据较多时，得到的运行结果会非常长，而且得到的运行结果也没太大的意义。所以，通过交叉连接的方式进行多表查询的这种方法并不常用，我们应该尽量避免这种查询。

查询 tb_course 表中的 id 字段和 tb_students_info 表中的 course_id 字段相等的内容， SQL 语句和运行结果如下：

mysql> select * from tb_course cross join tb_students_info where tb_course.id = tb_students_info.course_id;
+----+-------------+----+--------+------+------+--------+-----------+
| id | course_name | id | name   | age  | sex  | height | course_id |
+----+-------------+----+--------+------+------+--------+-----------+
|  1 | Java        |  1 | Dany   |   25 | M    |    160 |         1 |
|  2 | MySQL       |  2 | Green  |   23 | M    |    158 |         2 |
|  1 | Java        |  3 | Henry  |   23 | W    |    185 |         1 |
|  3 | Python      |  4 | Jane   |   22 | M    |    162 |         3 |
|  2 | MySQL       |  5 | Jim    |   24 | W    |    175 |         2 |
|  4 | Go          |  6 | John   |   21 | W    |    172 |         4 |
|  4 | Go          |  7 | Lily   |   22 | M    |    165 |         4 |
|  5 | C++         |  8 | Susan  |   23 | M    |    170 |         5 |
|  5 | C++         |  9 | Thomas |   22 | W    |    178 |         5 |
|  5 | C++         | 10 | Tom    |   23 | W    |    165 |         5 |
+----+-------------+----+--------+------+------+--------+-----------+
10 rows in set (0.00 sec)

查询每门课的课程对应的是那些人在学， SQL 语句和运行结果如下：

mysql> select tb_course.course_name,tb_students_info.name from tb_course cross join tb_students_info where tb_course.id = tb_students_info.course_id;
+-------------+--------+
| course_name | name   |
+-------------+--------+
| Java        | Dany   |
| MySQL       | Green  |
| Java        | Henry  |
| Python      | Jane   |
| MySQL       | Jim    |
| Go          | John   |
| Go          | Lily   |
| C++         | Susan  |
| C++         | Thomas |
| C++         | Tom    |
+-------------+--------+
10 rows in set (0.00 sec)

如果在交叉连接时使用 WHERE 子句，MySQL 会先生成两个表的笛卡尔积，然后再选择满足 WHERE 条件的记录。因此，表的数量较多时，交叉连接会非常非常慢。一般情况下不建议使用交叉连接。

在 MySQL 中，多表查询一般使用内连接和外连接，它们的效率要高于交叉连接.