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mysql进阶
二进制格式mysql安装
[root@lnh ~]# cd /usr/src/ [root@lnh 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 17:39:08-- 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)... 23.10.6.175, 2600:140b:2:a9b::2e31, 2600:140b:2:a93::2e31 Connecting to downloads.mysql.com (downloads.mysql.com)|23.10.6.175|: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 17:39:08-- 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)... 23.205.73.56 Connecting to cdn.mysql.com (cdn.mysql.com)|23.205.73.56|: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’ mysql-5.7.38-linux-gl 100%[=======================>] 643.57M 3.85MB/s in 3m 13s 2022-07-27 17:42:22 (3.34 MB/s) - ‘mysql-5.7.38-linux-glibc2.12-x86_64.tar.gz’ saved [674830866/674830866] //在www.mysql.com网站里面复制下载地址下载mysql包 [root@lnh src]# groupadd -r mysql [root@lnh src]# useradd -M -s /sbin/nologin -g mysql mysql //创建用户和组,使其所属主和组都是mysql [root@lnh src]# ls debug kernels mysql-5.7.38-linux-glibc2.12-x86_64.tar.gz [root@lnh src]# tar -xf mysql-5.7.38-linux-glibc2.12-x86_64.tar.gz -C /usr/local/ //将这个包解压到/usr/local这个目录 [root@lnh src]# ls /usr/local/ apache apr-util etc include lib64 mysql-5.7.38-linux-glibc2.12-x86_64 share apr bin games lib libexec sbin src //查看解压出来的东西 [root@lnh src]# cd /usr/local/ [root@lnh local]# ln -sv mysql-5.7.38-linux-glibc2.12-x86_64/ mysql 'mysql' -> 'mysql-5.7.38-linux-glibc2.12-x86_64/' [root@lnh local]# ll total 0 drwxr-xr-x. 14 root root 164 Jul 21 21:32 apache drwxr-xr-x. 6 root root 58 Jul 21 21:12 apr drwxr-xr-x. 5 root root 43 Jul 21 21:17 apr-util drwxr-xr-x. 2 root root 6 May 19 2020 bin drwxr-xr-x. 2 root root 6 May 19 2020 etc drwxr-xr-x. 2 root root 6 May 19 2020 games drwxr-xr-x. 2 root root 6 May 19 2020 include drwxr-xr-x. 2 root root 6 May 19 2020 lib drwxr-xr-x. 3 root root 17 Jul 19 16:13 lib64 drwxr-xr-x. 2 root root 6 May 19 2020 libexec lrwxrwxrwx. 1 root root 36 Jul 27 18:01 mysql -> mysql-5.7.38-linux-glibc2.12-x86_64/ drwxr-xr-x. 9 root root 129 Jul 27 17:54 mysql-5.7.38-linux-glibc2.12-x86_64 drwxr-xr-x. 2 root root 6 May 19 2020 sbin drwxr-xr-x. 5 root root 49 Jul 19 16:13 share drwxr-xr-x. 5 root root 145 Jul 21 21:19 src //映射头文件并查看 [root@lnh ~]# chown -R mysql.mysql /usr/local/mysql [root@lnh ~]# ll -d /usr/local/mysql lrwxrwxrwx. 1 mysql mysql 36 Jul 27 18:01 /usr/local/mysql -> mysql-5.7.38-linux-glibc2.12-x86_64/ //修改目录/usr/local/mysql的属主属组 [root@lnh ~]# echo 'export PATH=/usr/local/mysql/bin:$PATH' > /etc/profile.d/mysql.sh [root@lnh ~]# . /etc/profile.d/mysql.sh [root@lnh ~]# echo $PATH /usr/local/mysql/bin:/usr/local/apache/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin //添加环境变量 [root@lnh ~]# mkdir /opt/xbz [root@lnh ~]# chown -R mysql.mysql /opt/xbz/ [root@lnh ~]# ll /opt/ total 0 drwxr-xr-x. 2 mysql mysql 6 Jul 27 18:11 xbz //建立数据存放目录 [root@lnh ~]# /usr/local/mysql/bin/mysqld --initialize --user=mysql --datadir=/opt/xbz/ 2022-07-27T10:22:37.353570Z 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details). 2022-07-27T10:22:37.631892Z 0 [Warning] InnoDB: New log files created, LSN=45790 2022-07-27T10:22:37.676813Z 0 [Warning] InnoDB: Creating foreign key constraint system tables. 2022-07-27T10:22:37.737439Z 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: 09b3989b-0d96-11ed-b83a-000c2905f428. 2022-07-27T10:22:37.740356Z 0 [Warning] Gtid table is not ready to be used. Table 'mysql.gtid_executed' cannot be opened. 2022-07-27T10:22:38.119664Z 0 [Warning] A deprecated TLS version TLSv1 is enabled. Please use TLSv1.2 or higher. 2022-07-27T10:22:38.119698Z 0 [Warning] A deprecated TLS version TLSv1.1 is enabled. Please use TLSv1.2 or higher. 2022-07-27T10:22:38.120249Z 0 [Warning] CA certificate ca.pem is self signed. 2022-07-27T10:22:38.191019Z 1 [Note] A temporary password is generated for root@localhost: ?A=a;7Zken1x //初始化数据库 //请注意，这个命令的最后会生成一个临时密码，此处密码是?A=a;7Zken1x //再次注意，这个密码是随机的，你的不会跟我一样，一定要记住这个密码，因为一会登录时会用到,可以提前进行复制下来 [root@lnh ~]# vim /etc/my.cnf [root@lnh ~]# cat /etc/my.cnf [mysqld] basedir = /usr/local/mysql datadir = /opt/xbz socket = /tmp/mysql.sock port = 3306 pid-file = /opt/xbz/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 //生成配置文件 [root@lnh ~]# cp -a /usr/local/mysql/support-files/mysql.server /etc/init.d/mysqld [root@lnh ~]# sed -ri 's#^(basedir=).*#1/usr/local/mysql#g' /etc/init.d/mysqld [root@lnh ~]# sed -ri 's#^(datadir=).*#1/opt/xbz#g' /etc/init.d/mysqld //配置服务启动脚本 [root@lnh ~]# cd /usr/lib/systemd/system/ [root@lnh system]# cp sshd.service mysqld.service [root@lnh system]# vim mysqld.service [root@lnh system]# cat mysqld.service [Unit] Description=mysqld server daemon Documentation=man:sshd(8) man:sshd_config(5) After=network.target sshd-keygen.target Wants=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@lnh system]# systemctl daemon-reload [root@lnh system]# systemctl restart mysqld.service [root@lnh system]# ss -anlt | grep 3306 LISTEN 0 80 *:3306 *:* //将mysqld添加到system里面 [root@lnh ~]# mysql -uroot -p Enter password: //使用之前生成的临时密码 Welcome to the MySQL monitor. Commands end with ; or g. Your MySQL connection id is 2 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> set password = password('xbz123'); Query OK, 0 rows affected, 1 warning (0.00 sec) //修改密码 mysql> quit Bye [root@lnh ~]# mysql -uroot -pxbz123 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 6 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> //密码修改成功 错误解决
安装完mysql之后，mysql命令登录不成功，报错：mysql: error while loading shared libraries: libncurses.so.5: cannot open shared object file: No such file or directory
dnf -y install libncurses*可以解决
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 |
破解密码: [root@lnh ~]# vim /etc/my.cnf //修改/etc/my.cnf配置文件 [root@lnh ~]# cat /etc/my.cnf [mysqld] basedir = /usr/local/mysql datadir = /opt/xbz socket = /tmp/mysql.sock port = 3306 pid-file = /opt/xbz/mysql.pid user = mysql skip-name-resolve skip-grant-tables //添加这一行 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@lnh ~]# systemctl restart mysqld.service //重启服务 [root@lnh ~]# mysql //直接免密登录 Welcome to the MySQL monitor. Commands end with ; or g. Your MySQL connection id is 3 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) //查看默认数据库 mysql> use mysql; //进入数据库 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('lnh123') where user='root'; //修改密码 Query OK, 1 row affected, 1 warning (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 1 mysql> flush privileges; Query OK, 0 rows affected (0.00 sec) //刷新修改 [root@lnh ~]# vim /etc/my.cnf [root@lnh ~]# cat /etc/my.cnf [mysqld] basedir = /usr/local/mysql datadir = /opt/xbz socket = /tmp/mysql.sock port = 3306 pid-file = /opt/xbz/mysql.pid user = mysql skip-name-resolve //删除skip-grant-tables 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@lnh ~]# systemctl restart mysqld.service //重启服务 [root@lnh ~]# mysql -uroot -plnh123 //进行登录 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> quit Bye 多表联合查询
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。
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。
eg:
A:1,2
B: 3,4,5
A X B={(1,3), (1,4), (1,5), (2,3), (2,4), (2,5) };
B X A={(3,1), (3,2), (4,1), (4,2), (5,1), (5,2) };
就是取两个范围内的交集
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];
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 | 男 | 160 | 1 |
| 2 | Green | 23 | 男 | 158 | 2 |
| 3 | Henry | 23 | 女 | 185 | 1 |
| 4 | Jane | 22 | 男 | 162 | 3 |
| 5 | Jim | 24 | 女 | 175 | 2 |
| 6 | John | 21 | 女 | 172 | 4 |
| 7 | Lily | 22 | 男 | 165 | 4 |
| 8 | Susan | 23 | 男 | 170 | 5 |
| 9 | Thomas | 22 | 女 | 178 | 5 |
| 10 | Tom | 23 | 女 | 165 | 5 |
+----+--------+------+------+--------+-----------+
10 rows in set (0.00 sec)
[root@lnh ~]# mysql -uroot -p Enter password: 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> create database lnh character set utf8; //创建数据库 Query OK, 1 row affected (0.00 sec) mysql> show databases; //查看数据库 +--------------------+ | Database | +--------------------+ | information_schema | | lnh | | mysql | | performance_schema | | sys | | tushanbu | +--------------------+ 6 rows in set (0.00 sec) mysql> use lnh; //进入数据库 Database changed mysql> create table tb_students_info(id int primary key auto_increment,name varchar(20),age tinyint,sex varchar(6),height int,course_id tinyint) charset utf8; Query OK, 0 rows affected (0.01 sec) mysql> insert into tb_students_info(name,age,sex,height,course_id) values('Dany',25,'男',160,1),('Green',23,'男',158,2),('Henry',23,'女',185,1),('Jane',22,'男',162,3),('Jin',21,'女',172,4),('John',21,'女',172,4),('Lily',22,'男',165,4),('Susan',23,'男',170,5),('Thomas',22,'女',178,5),('Tom',23,'女',165,5); Query OK, 10 rows affected (0.00 sec) Records: 10 Duplicates: 0 Warnings: 0 //插入表 mysql> select * from tb_students_info; //查看这个表的数据 +----+--------+------+------+--------+-----------+ | id | name | age | sex | height | course_id | +----+--------+------+------+--------+-----------+ | 1 | Dany | 25 | 男 | 160 | 1 | | 2 | Green | 23 | 男 | 158 | 2 | | 3 | Henry | 23 | 女 | 185 | 1 | | 4 | Jane | 22 | 男 | 162 | 3 | | 5 | Jin | 21 | 女 | 172 | 4 | | 6 | John | 21 | 女 | 172 | 4 | | 7 | Lily | 22 | 男 | 165 | 4 | | 8 | Susan | 23 | 男 | 170 | 5 | | 9 | Thomas | 22 | 女 | 178 | 5 | | 10 | Tom | 23 | 女 | 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)
mysql> create table tb_course(id int primary key auto_increment,course_name varchar(20)) charset utf8; Query OK, 0 rows affected (0.02 sec) mysql> insert into tb_course(id,course_name) values(1,'java'),(2,'mysql'),(3,'python')),(4,'Go'),(5,'c++'); Query OK, 5 rows affected (0.01 sec) Records: 5 Duplicates: 0 Warnings: 0 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 | 男 | 160 | 1 | | 2 | mysql | 1 | Dany | 25 | 男 | 160 | 1 | | 3 | python | 1 | Dany | 25 | 男 | 160 | 1 | | 4 | Go | 1 | Dany | 25 | 男 | 160 | 1 | | 5 | c++ | 1 | Dany | 25 | 男 | 160 | 1 | | 1 | java | 2 | Green | 23 | 男 | 158 | 2 | | 2 | mysql | 2 | Green | 23 | 男 | 158 | 2 | | 3 | python | 2 | Green | 23 | 男 | 158 | 2 | | 4 | Go | 2 | Green | 23 | 男 | 158 | 2 | | 5 | c++ | 2 | Green | 23 | 男 | 158 | 2 | | 1 | java | 3 | Henry | 23 | 女 | 185 | 1 | | 2 | mysql | 3 | Henry | 23 | 女 | 185 | 1 | | 3 | python | 3 | Henry | 23 | 女 | 185 | 1 | | 4 | Go | 3 | Henry | 23 | 女 | 185 | 1 | | 5 | c++ | 3 | Henry | 23 | 女 | 185 | 1 | | 1 | java | 4 | Jane | 22 | 男 | 162 | 3 | | 2 | mysql | 4 | Jane | 22 | 男 | 162 | 3 | | 3 | python | 4 | Jane | 22 | 男 | 162 | 3 | | 4 | Go | 4 | Jane | 22 | 男 | 162 | 3 | | 5 | c++ | 4 | Jane | 22 | 男 | 162 | 3 | | 1 | java | 5 | Jin | 21 | 女 | 172 | 4 | | 2 | mysql | 5 | Jin | 21 | 女 | 172 | 4 | | 3 | python | 5 | Jin | 21 | 女 | 172 | 4 | | 4 | Go | 5 | Jin | 21 | 女 | 172 | 4 | | 5 | c++ | 5 | Jin | 21 | 女 | 172 | 4 | | 1 | java | 6 | John | 21 | 女 | 172 | 4 | | 2 | mysql | 6 | John | 21 | 女 | 172 | 4 | | 3 | python | 6 | John | 21 | 女 | 172 | 4 | | 4 | Go | 6 | John | 21 | 女 | 172 | 4 | | 5 | c++ | 6 | John | 21 | 女 | 172 | 4 | | 1 | java | 7 | Lily | 22 | 男 | 165 | 4 | | 2 | mysql | 7 | Lily | 22 | 男 | 165 | 4 | | 3 | python | 7 | Lily | 22 | 男 | 165 | 4 | | 4 | Go | 7 | Lily | 22 | 男 | 165 | 4 | | 5 | c++ | 7 | Lily | 22 | 男 | 165 | 4 | | 1 | java | 8 | Susan | 23 | 男 | 170 | 5 | | 2 | mysql | 8 | Susan | 23 | 男 | 170 | 5 | | 3 | python | 8 | Susan | 23 | 男 | 170 | 5 | | 4 | Go | 8 | Susan | 23 | 男 | 170 | 5 | | 5 | c++ | 8 | Susan | 23 | 男 | 170 | 5 | | 1 | java | 9 | Thomas | 22 | 女 | 178 | 5 | | 2 | mysql | 9 | Thomas | 22 | 女 | 178 | 5 | | 3 | python | 9 | Thomas | 22 | 女 | 178 | 5 | | 4 | Go | 9 | Thomas | 22 | 女 | 178 | 5 | | 5 | c++ | 9 | Thomas | 22 | 女 | 178 | 5 | | 1 | java | 10 | Tom | 23 | 女 | 165 | 5 | | 2 | mysql | 10 | Tom | 23 | 女 | 165 | 5 | | 3 | python | 10 | Tom | 23 | 女 | 165 | 5 | | 4 | Go | 10 | Tom | 23 | 女 | 165 | 5 | | 5 | c++ | 10 | Tom | 23 | 女 | 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。
例 2
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_students_info.course_id =tb_course.id; +----+-------------+----+--------+------+------+--------+-----------+ | id | course_name | id | name | age | sex | height | course_id | +----+-------------+----+--------+------+------+--------+-----------+ | 1 | java | 1 | Dany | 25 | 男 | 160 | 1 | | 2 | mysql | 2 | Green | 23 | 男 | 158 | 2 | | 1 | java | 3 | Henry | 23 | 女 | 185 | 1 | | 3 | python | 4 | Jane | 22 | 男 | 162 | 3 | | 4 | Go | 5 | Jin | 21 | 女 | 172 | 4 | | 4 | Go | 6 | John | 21 | 女 | 172 | 4 | | 4 | Go | 7 | Lily | 22 | 男 | 165 | 4 | | 5 | c++ | 8 | Susan | 23 | 男 | 170 | 5 | | 5 | c++ | 9 | Thomas | 22 | 女 | 178 | 5 | | 5 | c++ | 10 | Tom | 23 | 女 | 165 | 5 | +----+-------------+----+--------+------+------+--------+-----------+ 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。
