mysql進階
目錄
- mysql進階
- 二進制格式mysql安裝
- 錯誤解決
- mysql組態檔
- 多表聯合查詢
- 什么是多表聯合查詢
- 笛卡爾積
- 交叉連接
二進制格式mysql安裝
[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網站里面復制下載地址下載mysql包
[root@lnh src]# groupadd -r mysql
[root@lnh src]# useradd -M -s /sbin/nologin -g mysql mysql
//創建用戶和組,使其所屬主和組都是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/ //將這個包解壓到/usr/local這個目錄
[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
//查看解壓出來的東西
[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
//映射頭檔案并查看
[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/
//修改目錄/usr/local/mysql的屬主屬組
[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
//添加環境變數
[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
//建立資料存放目錄
[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
//初始化資料庫
//請注意，這個命令的最后會生成一個臨時密碼，此處密碼是?A=a;7Zken1x
//再次注意，這個密碼是隨機的，你的不會跟我一樣，一定要記住這個密碼，因為一會登錄時會用到,可以提前進行復制下來
[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
//生成組態檔
[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
//配置服務啟動腳本
[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 *:*
//將mysqld添加到system里面
[root@lnh ~]# mysql -uroot -p
Enter password: //使用之前生成的臨時密碼
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)
//修改密碼
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>
//密碼修改成功
錯誤解決
安裝完mysql之后，mysql命令登錄不成功，報錯：mysql: error while loading shared libraries: libncurses.so.5: cannot open shared object file: No such file or directory
dnf -y install libncurses*可以解決
mysql組態檔
mysql的組態檔為/etc/my.cnf
組態檔查找次序：若在多個組態檔中均有設定，則最后找到的最終生效
/etc/my.cnf --> /etc/mysql/my.cnf --> --default-extra-file=/PATH/TO/CONF_FILE --> ~/.my.cnf
mysql常用組態檔引數：
| 引數 | 說明 |
|---|---|
| port = 3306 | 設定監聽埠 |
| socket = /tmp/mysql.sock | 指定套接字檔案位置 |
| basedir = /usr/local/mysql | 指定MySQL的安裝路徑 |
| datadir = /data/mysql | 指定MySQL的資料存放路徑 |
| pid-file = /data/mysql/mysql.pid | 指定行程ID檔案存放路徑 |
| user = mysql | 指定MySQL以什么用戶的身份提供服務 |
| skip-name-resolve | 禁止MySQL對外部連接進行DNS決議 使用這一選項可以消除MySQL進行DNS決議的時間， 若開啟該選項，則所有遠程主機連接授權都要使用IP地址方 式否則MySQL將無法正常處理連接請求 |
破解密碼:
[root@lnh ~]# vim /etc/my.cnf //修改/etc/my.cnf組態檔
[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 //添加這一行
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
//重啟服務
[root@lnh ~]# mysql //直接免密登錄
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)
//查看默認資料庫
mysql> use mysql; //進入資料庫
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'; //修改密碼
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)
//重繪修改
[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 //洗掉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 //重啟服務
[root@lnh ~]# mysql -uroot -plnh123 //進行登錄
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
多表聯合查詢
什么是多表聯合查詢
前面所講的查詢陳述句都是針對一個表的，但是在關系型資料庫中，表與表之間是有聯系的，所以在實際應用中，經常使用多表查詢，多表查詢就是同時查詢兩個或兩個以上的表，在 MySQL 中，多表查詢主要有交叉連接、內連接、外連接、分組查詢與子查詢等5種，
笛卡爾積
交叉連接(CROSS JOIN)：有兩種，顯式的和隱式的2種，一般用來回傳連接表的笛卡爾積，
笛卡爾積（Cartesian product）是指兩個集合 X 和 Y 的乘積，
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) };
就是取兩個范圍內的交集
交叉連接
交叉連接的語法格式如下：
SELECT <欄位名> FROM <表1> CROSS JOIN <表2> [WHERE子句];
或
SELECT <欄位名> FROM <表1>, <表2> [WHERE子句];
1）查詢 tb_students_info 表中的資料，SQL 陳述句和運行結果如下：
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; //創建資料庫
Query OK, 1 row affected (0.00 sec)
mysql> show databases; //查看資料庫
+--------------------+
| Database |
+--------------------+
| information_schema |
| lnh |
| mysql |
| performance_schema |
| sys |
| tushanbu |
+--------------------+
6 rows in set (0.00 sec)
mysql> use lnh; //進入資料庫
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; //查看這個表的資料
+----+--------+------+------+--------+-----------+
| 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）查詢 tb_course 表中的資料，SQL 陳述句和運行結果如下：
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 查詢出兩張表中的笛卡爾積，SQL 陳述句和運行結果如下：
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)
由運行結果可以看出，tb_course 和 tb_students_info 表交叉連接查詢后，回傳了 50 條記錄，可以想象，當表中的資料較多時，得到的運行結果會非常長，而且得到的運行結果也沒太大的意義，所以，通過交叉連接的方式進行多表查詢的這種方法并不常用，我們應該盡量避免這種查詢，
例 2
查詢 tb_course 表中的 id 欄位和 tb_students_info 表中的 course_id 欄位相等的內容， SQL 陳述句和運行結果如下：
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)
如果在交叉連接時使用 WHERE 子句，MySQL 會先生成兩個表的笛卡爾積，然后再選擇滿足 WHERE 條件的記錄，因此，表的數量較多時，交叉連接會非常非常慢，一般情況下不建議使用交叉連接，
在 MySQL 中，多表查詢一般使用內連接和外連接，它們的效率要高于交叉連接，
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