mysql进阶-二进制安装mysql，交叉连接
mysql进阶
二进制格式安装mysql
//下载二进制格式的mysql软件包
[root@localhost ~]# cd /usr/src/
[root@localhost src]# wget https://downloads.mysql.com/archives/get/p/23/file/mysql-5.7.38-linux-glibc2.12-x86_64.tar.gz
[root@localhost src]# ls
debug kernels mysql-5.7.38-linux-glibc2.12-x86_64.tar.gz
//创建用户和组
[root@localhost src]# id mysql //看看系统有没有用户
id: 'mysql': no such user
[root@localhost src]# useradd -M -r -s /sbin/nologin mysql //-M，不建立用户家目录，-r，建立系统帐号，-s，不允许登录。
[root@localhost src]# id mysql
uid=995(mysql) gid=992(mysql) groups=992(mysql)
//解压软件包至/usr/local/
[root@localhost src]# tar xf mysql-5.7.38-linux-glibc2.12-x86_64.tar.gz -C /usr/local/
[root@localhost src]# cd /usr/local/
[root@localhost local]# ls
bin games lib libexec sbin src
etc include lib64 mysql-5.7.38-linux-glibc2.12-x86_64 share
//给mysql目录做个软连接
[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]# 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 Jul 26 09:28 lib64
drwxr-xr-x. 2 root root 6 Jun 22 2021 libexec
lrwxrwxrwx. 1 root root 36 Jul 27 15:35 mysql -> mysql-5.7.38-linux-glibc2.12-x86_64/
drwxr-xr-x. 9 root root 129 Jul 27 15:33 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 Jul 26 09:28 share
drwxr-xr-x. 2 root root 6 Jun 22 2021 src
//修改目录/usr/local/mysql的属主属组
[root@localhost local]# chown -R mysql.mysql /usr/local/mysql
[root@localhost local]# ll mysql -d
lrwxrwxrwx. 1 mysql mysql 36 Jul 27 15:35 mysql -> mysql-5.7.38-linux-glibc2.12-x86_64/
//添加环境变量
[root@localhost local]# cd mysql
[root@localhost mysql]# ls
LICENSE README bin docs include lib man share support-files
[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]# echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin:/usr/local/mysql/bin
[root@localhost bin]# which mysql
/usr/local/mysql/bin/mysql
//添加头文件
[root@localhost bin]# cd
[root@localhost ~]# cd /usr/local/mysql
[root@localhost mysql]# ls
LICENSE README bin docs include lib man share support-files
[root@localhost mysql]# ln -sv /usr/local/mysql/include/ /usr/include/mysql
'/usr/include/mysql' -> '/usr/local/mysql/include/'
//配置lib和man
[root@localhost mysql]# vim /etc/ld.so.conf.d/mysql.conf
[root@localhost mysql]# cat /etc/ld.so.conf.d/mysql.conf
/usr/local/mysql/lib/
[root@localhost mysql]# ldconfig
[root@localhost mysql]# vim /etc/man_db.conf //进入后向下翻一些
MANDATORY_MANPATH /usr/man
MANDATORY_MANPATH /usr/share/man
MANDATORY_MANPATH /usr/local/share/man
MANDATORY_MANPATH /usr/local/mysql/man //添加此条
//建立数据存放目录
[root@localhost ~]# mkdir /opt/data
[root@localhost ~]# chown -R mysql.mysql /opt/data/
[root@localhost ~]# ll /opt/
total 0
drwxr-xr-x. 2 mysql mysql 6 Jul 27 15:52 data
//初始化数据库
[root@localhost ~]# mysqld --initialize --user mysql --datadir /opt/data/
2022-07-27T07:54:56.772874Z 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details).
2022-07-27T07:54:57.178265Z 0 [Warning] InnoDB: New log files created, LSN=45790
2022-07-27T07:54:57.252461Z 0 [Warning] InnoDB: Creating foreign key constraint system tables.
2022-07-27T07:54:57.268967Z 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: 68736795-0d81-11ed-be52-000c29160299.
2022-07-27T07:54:57.270174Z 0 [Warning] Gtid table is not ready to be used. Table 'mysql.gtid_executed' cannot be opened.
2022-07-27T07:54:57.521327Z 0 [Warning] A deprecated TLS version TLSv1 is enabled. Please use TLSv1.2 or higher.
2022-07-27T07:54:57.521354Z 0 [Warning] A deprecated TLS version TLSv1.1 is enabled. Please use TLSv1.2 or higher.
2022-07-27T07:54:57.521842Z 0 [Warning] CA certificate ca.pem is self signed.
2022-07-27T07:54:57.577731Z 1 [Note] A temporary password is generated for root@localhost: iB6D>oGmVUy3
//请注意，这个命令的最后会生成一个临时密码，此处密码是iB6D>oGmVUy3
//生成配置文件
[root@localhost ~]# vim /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
//配置服务启动脚本
//第一种方法centos6的启动方式，不推荐使用咱用centos8还用6的方法比较傻
[root@localhost ~]# cp -a /usr/local/mysql/support-files/mysql.server /etc/init.d/mysqld
[root@localhost ~]# vim /etc/init.d/mysqld //进入搜basedir，给basedir=后面添加下面内容
basedir=/usr/local/mysql
datadir=/opt/data
[root@localhost ~]# service mysqld start
Starting MySQL.Logging to '/opt/data/localhost.localdomain.err'.
SUCCESS!
[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 *:*
[root@localhost ~]# service mysqld stop
Shutting down MySQL. SUCCESS!
[root@localhost ~]# ss -antlState 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 [::]:*
//第二种方法
[root@localhost ~]# rm -rf /etc/init.d/mysqld //删除刚刚创建的一种启动方式
[root@localhost ~]# service mysqld start //删除后已经无法用之前的方式启动
Redirecting to /bin/systemctl start mysqld.service
Failed to start mysqld.service: Unit mysqld.service not found.
[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 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 ~]# mysql -uroot -p'iB6D>oGmVUy3'
mysql: error while loading shared libraries: libncurses.so.5: cannot open shared object file: No such file or directory
//缺少libncurses.so.5相关的软件包，查询并下载就好
[root@localhost ~]# dnf whatprovides libncurses.so.5
Failed to set locale, defaulting to C.UTF-8
Last metadata expiration check: 2:22:12 ago on Wed Jul 27 14:16:59 2022.
ncurses-compat-libs-6.1-7.20180224.el8.i686 : Ncurses compatibility libraries
Repo : baseos
Matched from:
Provide : libncurses.so.5
ncurses-compat-libs-6.1-9.20180224.el8.i686 : Ncurses compatibility libraries
Repo : baseos
Matched from:
Provide : libncurses.so.5
[root@localhost ~]# dnf -y install ncurses-compat-libs
[root@localhost ~]# mysql -uroot -p'iB6D>oGmVUy3'
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>
//登录成功后修改密码
mysql> set password = password('123456');
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> exit
Bye
[root@localhost ~]# mysql -uroot -p123456
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 4
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>
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 |
|---|---|
| 端口 = 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 |
| 跳过名称解析 | 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的
1、修改/etc/my.cnf配置文件
[root@localhost ~]# vim /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
2、重启mysql服务
[root@localhost ~]# systemctl restart mysqld
3、mysql 登入修改密码
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
+--------------------+
4 rows in set (0.00 sec)
mysql> use mysql;
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
4、编辑/etc/my.cnf配置文，删除skip-grant-tables，然后重启启动mysql服务
[root@localhost ~]# vim /etc/my.cnf
[root@localhost ~]# systemctl restart mysqld
5、使用新密码登入mysql测试
[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>
多表联合查询
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。
2.交叉连接
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（笛卡尔积）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。
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。
交叉连接可以查询两个或两个以上的表，为了更好的理解，我们就讲解两个表的交叉连接查询。
//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.
