DML、DQL与单表多表的增删改查20190215下午

分类：Mariadb作者：编程之家用户

MariaDB [db1]> select * from student;
+----+------+-----+--------+-------+
| id | name | age | gender | phone |
+----+------+-----+--------+-------+
| 1 | a | 20 | m | 119 |
| 2 | b | 20 | m | 120 |
| 3 | c | 20 | m | 110 |
| 4 | d | 20 | m | 135 |
| 5 | e | 20 | m | 121 |
| 6 | f | 20 | m | 100 |
+----+------+-----+--------+-------+
6 rows in set (0.00 sec)
修改其中一行内容：
MariaDB [db1]> update student set phone='1101' where name='c';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0

MariaDB [db1]> select * from student;
+----+------+-----+--------+-------+
| id | name | age | gender | phone |
+----+------+-----+--------+-------+
| 1 | a | 20 | m | 119 |
| 2 | b | 20 | m | 120 |
| 3 | c | 20 | m | 1101 |
| 4 | d | 20 | m | 135 |
| 5 | e | 20 | m | 121 |
| 6 | f | 20 | m | 100 |
+----+------+-----+--------+-------+
6 rows in set (0.00 sec)
当我们加上-U登陆后，若要修改文件而没有使用where语句，系统将会爆出提醒：
[root@centos7 ~]# mysql -S /mysql/3306/socket/mysql.sock -p12345gxy -U
Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 6
Server version: 5.5.60-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)]> use db1;
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
MariaDB [db1]> select * from student;
+----+------+-----+--------+-------+
| id | name | age | gender | phone |
+----+------+-----+--------+-------+
| 1 | a | 20 | m | 119 |
| 2 | b | 20 | m | 120 |
| 3 | c | 20 | m | 1101 |
| 4 | d | 20 | m | 135 |
| 5 | e | 20 | m | 121 |
| 6 | f | 20 | m | 100 |
+----+------+-----+--------+-------+
6 rows in set (0.00 sec)

MariaDB [db1]> update student set phone='100' ;
ERROR 1175 (HY000): You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column
MariaDB [db1]> （你正在使用安全登陆模式）
我们去修改客户端的配置文件:
[root@centos7 ~]# vim /etc/my.cnf.d/mysql-clients.cnf

These groups are read by MariaDB command-line tools

Use it for options that affect only one utility

[mysql]
safe-updates 加入左边的内容。
再去更新数据，不写where语句，系统是不会让更新的，如下：
Database changed
MariaDB [db1]> update student set phone='120';
ERROR 1175 (HY000): You are using safe update mode and you tried to update a table without a WHERE that uses a KEY column
MariaDB [db1]>
删表：MariaDB [db1]> delete from student where id=6;
Query OK, 1 row affected (0.00 sec)

MariaDB [db1]> select * from student;
+----+------+-----+--------+-------+
| id | name | age | gender | phone |
+----+------+-----+--------+-------+
| 1 | a | 20 | m | 119 |
| 2 | b | 20 | m | 120 |
| 3 | c | 20 | m | 1101 |
| 4 | d | 20 | m | 135 |
| 5 | e | 20 | m | 121 |
+----+------+-----+--------+-------+
5 rows in set (0.00 sec)
使用“ delete ”会记录日志，而使用“ truncate ”会更快速彻底，它是清空表，不记录日志：
给表设置别名，如下：

MariaDB [hellodb]> select stuID as 编号,name 姓名,Age as 年龄 from students;

+--------+---------------+--------+
| 编号 | 姓名 | 年龄 |
+--------+---------------+--------+
| 1 | Shi Zhongyu | 22 |
| 2 | Shi Potian | 22 |
| 3 | Xie Yanke | 53 |
| 4 | Ding Dian | 32 |
| 5 | Yu Yutong | 26 |
| 6 | Shi Qing | 46 |
| 7 | Xi Ren | 19 |
| 8 | Lin Daiyu | 17 |
| 9 | Ren Yingying | 20 |
| 10 | Yue Lingshan | 19 |
| 11 | Yuan Chengzhi | 23 |
| 12 | Wen Qingqing | 19 |
| 13 | Tian Boguang | 33 |
| 14 | Lu Wushuang | 17 |
| 15 | Duan Yu | 19 |
| 16 | Xu Zhu | 21 |
| 17 | Lin Chong | 25 |
| 18 | Hua Rong | 23 |
| 19 | Xue Baochai | 18 |
| 20 | Diao Chan | 19 |
| 21 | Huang Yueying | 22 |
| 22 | Xiao Qiao | 20 |
| 23 | Ma Chao | 23 |
| 24 | Xu Xian | 27 |
| 25 | Sun Dasheng | 100 |
+--------+---------------+--------+
25 rows in set (0.00 sec)
count函数的使用：
MariaDB [hellodb]> select count() from students;
+----------+
| count() |
+----------+
| 25 |
+----------+
1 row in set (0.00 sec)

MariaDB [hellodb]> select count() as 记录数 from students;
+-----------+
| 记录数 |
+-----------+
| 25 |
+-----------+
1 row in set (0.00 sec)
挑出特定范围的内容：
MariaDB [hellodb]> select from students where age > 30;
+-------+--------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+--------------+-----+--------+---------+-----------+
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+--------------+-----+--------+---------+-----------+
5 rows in set (0.00 sec)
使用select命令把表中有多少行就将23的结果显示多少遍：
MariaDB [hellodb]> select 23 from students;
+-----+
| 2*3 |
+-----+
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
| 6 |
+-----+
25 rows in set (0.00 sec)
字符串和乘积均可显示：
MariaDB [hellodb]> select 'abcd', stuid from students;
+------+-------+
| abcd | stuid |
+------+-------+
| abcd | 1 |
| abcd | 2 |
| abcd | 3 |
| abcd | 4 |
| abcd | 5 |
| abcd | 6 |
| abcd | 7 |
| abcd | 8 |
| abcd | 9 |
| abcd | 10 |
| abcd | 11 |
| abcd | 12 |
| abcd | 13 |
| abcd | 14 |
| abcd | 15 |
| abcd | 16 |
| abcd | 17 |
| abcd | 18 |
| abcd | 19 |
| abcd | 20 |
| abcd | 21 |
| abcd | 22 |
| abcd | 23 |
| abcd | 24 |
| abcd | 25 |
+------+-------+
25 rows in set (0.00 sec)

MariaDB [hellodb]> select 23, stuid from students;
+-----+-------+
| 23 | stuid |
+-----+-------+
| 6 | 1 |
| 6 | 2 |
| 6 | 3 |
| 6 | 4 |
| 6 | 5 |
| 6 | 6 |
| 6 | 7 |
| 6 | 8 |
| 6 | 9 |
| 6 | 10 |
| 6 | 11 |
| 6 | 12 |
| 6 | 13 |
| 6 | 14 |
| 6 | 15 |
| 6 | 16 |
| 6 | 17 |
| 6 | 18 |
| 6 | 19 |
| 6 | 20 |
| 6 | 21 |
| 6 | 22 |
| 6 | 23 |
| 6 | 24 |
| 6 | 25 |
+-----+-------+
25 rows in set (0.00 sec)
select命令还可以这样做：
MariaDB [hellodb]> select from students where age between 20 and 30;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
12 rows in set (0.00 sec)
select命令查看某列内容为空的的字符：（某id is NULL）
MariaDB [hellodb]> select from students where teacherid is NULL;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
20 rows in set (0.00 sec)
MariaDB [hellodb]> select from students where teacherid is not NULL;
+-------+-------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+-------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
+-------+-------------+-----+--------+---------+-----------+
5 rows in set (0.00 sec)
select命令查询特定字段：
MariaDB [hellodb]> select from students where classid in (1,2,5);
+-------+--------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+--------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
+-------+--------------+-----+--------+---------+-----------+
8 rows in set (0.00 sec)
新create一张表，insert命令写入数据：
MariaDB [hellodb]> create table user (id int,name char(10),password varchar(10));
Query OK, 0 rows affected (0.00 sec)

MariaDB [hellodb]> insert user values(1,'admin','xingxiaoyatongxue'),(2,'mage','xiqukeke');
Query OK, 2 rows affected, 1 warning (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 1

MariaDB [hellodb]> select from user;
+------+-------+------------+
| id | name | password |
+------+-------+------------+
| 1 | admin | xingxiaoya |
| 2 | mage | xiqukeke |
+------+-------+------------+
2 rows in set (0.00 sec)
用户登录时，只有输入正确的用户名及口令，才能登陆：
MariaDB [hellodb]> select from user where name='mage' and password='xiqukeke';
+------+------+----------+
| id | name | password |
+------+------+----------+
| 2 | mage | xiqukeke |
+------+------+----------+
1 row in set (0.00 sec)
我们修改服务器端的配置文件：
[root@centos7 ~]# vim /mysql/3306/etc/my.cnf
[mysqld]

character-set-server=utf8mb4 将其加入

port=3306
datadir=/mysql/3306/data
socket=/mysql/3306/socket/mysql.sock
我们修改客户端的配置文件：
[root@centos7 ~]# vim /etc/my.cnf.d/mysql-clients.cnf

These groups are read by MariaDB command-line tools

Use it for options that affect only one utility

[mysql]

default-character-set=utf8mb4 将其加入

[mysql_upgrade]
重启服务：
[root@centos7 ~]# /mysql/3306/bin/mysqld restart
Restarting MySQL...
Stoping MySQL...
Starting MySQL...
[root@centos7 ~]# mysql -S /mysql/3306/socket/mysql.sock -p12345gxy
Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 1
Server version: 5.5.60-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)]> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| db1 |
| hellodb |
| mysql |
| performance_schema |
| test |
| test1 |
+--------------------+
7 rows in set (0.00 sec)
修改完配置文件后，新create一张表，查看其字符集类型均是utf8mb4：
MariaDB [(none)]> create database db2;
Query OK, 1 row affected (0.00 sec)

MariaDB [(none)]> show create database db2;
+----------+-----------------------------------------------------------------+
| Database | Create Database |
+----------+-----------------------------------------------------------------+
| db2 | CREATE DATABASE db2 /!40100 DEFAULT CHARACTER SET utf8mb4 / |
+----------+-----------------------------------------------------------------+
1 row in set (0.00 sec)

MariaDB [(none)]> show variables like 'character%';
+--------------------------+----------------------------+
| Variable_name | Value |
+--------------------------+----------------------------+
| character_set_client | utf8mb4 |
| character_set_connection | utf8mb4 |
| character_set_database | utf8mb4 |
| character_set_filesystem | binary |
| character_set_results | utf8mb4 |
| character_set_server | utf8mb4 |
| character_set_system | utf8 |
| character_sets_dir | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+
8 rows in set (0.00 sec)
在db2中create一个t1，查看其字符集类型：
MariaDB [(none)]> use db2;
Database changed
MariaDB [db2]> create table t1 (id int,name char(20));
Query OK, 0 rows affected (0.01 sec)

MariaDB [db2]> select * from t1;
Empty set (0.00 sec)

MariaDB [db2]> show create table t1;
+-------+-------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+-------------------------------------------------------------------------------------------------------------------------+
| t1 | CREATE TABLE t1 (
id int(11) DEFAULT NULL,
name char(20) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 |
+-------+-------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
字符串去重：
MariaDB [hellodb]> select distinct gender from students;
+--------+
| gender |
+--------+
| M |
| F |
+--------+
2 rows in set (0.01 sec)
模糊匹配：
MariaDB [hellodb]> select * from students where name like 's%';
+-------+-------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+-------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+-------------+-----+--------+---------+-----------+
4 rows in set (0.00 sec)

MariaDB [hellodb]> select from students where name like '%in%';
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
+-------+---------------+-----+--------+---------+-----------+
8 rows in set (0.00 sec)
查询只有四个字母的名字：
MariaDB [hellodb]> select from user where name like '____';
+------+------+----------+
| id | name | password |
+------+------+----------+
| 2 | mage | xiqukeke |
+------+------+----------+
1 row in set (0.00 sec)
group by 分组：
MariaDB [hellodb]> select avg(age) from students group by gender;
+----------+
| avg(age) |
+----------+
| 19.0000 |
| 33.0000 |
+----------+
2 rows in set (0.00 sec)
加上gender，才能看出是m或f：
MariaDB [hellodb]> select gender,avg(age) from students group by gender;
+--------+----------+
| gender | avg(age) |
+--------+----------+
| F | 19.0000 |
| M | 33.0000 |
+--------+----------+
2 rows in set (0.00 sec)
设置别名为汉字：
MariaDB [hellodb]> select gender,avg(age) 平均年龄 from students group by gender;
+--------+--------------+
| gender | 平均年龄 |
+--------+--------------+
| F | 19.0000 |
| M | 33.0000 |
+--------+--------------+
2 rows in set (0.00 sec)
统计最大值，最小值，总和：
MariaDB [hellodb]> select gender,max(age) from students group by gender;
+--------+----------+
| gender | max(age) |
+--------+----------+
| F | 22 |
| M | 100 |
+--------+----------+
2 rows in set (0.00 sec)

MariaDB [hellodb]> select gender,min(age) from students group by gender;
+--------+----------+
| gender | min(age) |
+--------+----------+
| F | 17 |
| M | 19 |
+--------+----------+
2 rows in set (0.00 sec)

MariaDB [hellodb]> select gender,sum(age) from students group by gender;
+--------+----------+
| gender | sum(age) |
+--------+----------+
| F | 190 |
| M | 495 |
+--------+----------+
2 rows in set (0.00 sec)
group by 与having的语法使用：
MariaDB [hellodb]> select gender,avg(age) 平均年龄 from students group by gender having gender='M';
+--------+--------------+
| gender | 平均年龄 |
+--------+--------------+
| M | 33.0000 |
+--------+--------------+
1 row in set (0.00 sec)
“ order by ”正向从小到大排序：
MariaDB [hellodb]> select from students order by age;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec)
“ desc ”倒序排序：
MariaDB [hellodb]> select from students order by age desc;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec)
“ desc ”工具排序，在“class ”后加“ - ”，进正序排序，有空值就会被放于最后：
MariaDB [hellodb]> select from students order by -classid desc;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec)
limit ”只显示限定的内容：
“MariaDB [hellodb]> select from students order by age limit 3;
+-------+-------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+-------------+-----+--------+---------+-----------+
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
+-------+-------------+-----+--------+---------+-----------+
3 rows in set (0.01 sec)
先对班级排序，再对年龄排序，结果看起来比较齐整：
MariaDB [hellodb]> select * from students order by classid, age;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 24 | Xu Xian | 27 | M | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 13 | Tian Boguang | 33 | M | 2 | NULL |
| 3 | Xie Yanke | 53 | M | 2 | 16 |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 4 | Ding Dian | 32 | M | 4 | 4 |
| 6 | Shi Qing | 46 | M | 5 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
+-------+---------------+-----+--------+---------+-----------+
25 rows in set (0.00 sec)

多表操作：

使用“ union ”将多表组合在一起：
MariaDB [hellodb]> select stuid,name from students 学生的序号和名字
-> union
-> select tid,name from teachers;教师的序号和名字
+-------+---------------+
| stuid | name |
+-------+---------------+
| 1 | Shi Zhongyu |
| 2 | Shi Potian |
| 3 | Xie Yanke |
| 4 | Ding Dian |
| 5 | Yu Yutong |
| 6 | Shi Qing |
| 7 | Xi Ren |
| 8 | Lin Daiyu |
| 9 | Ren Yingying |
| 10 | Yue Lingshan |
| 11 | Yuan Chengzhi |
| 12 | Wen Qingqing |
| 13 | Tian Boguang |
| 14 | Lu Wushuang |
| 15 | Duan Yu |
| 16 | Xu Zhu |
| 17 | Lin Chong |
| 18 | Hua Rong |
| 19 | Xue Baochai |
| 20 | Diao Chan |
| 21 | Huang Yueying |
| 22 | Xiao Qiao |
| 23 | Ma Chao |
| 24 | Xu Xian |
| 25 | Sun Dasheng |
| 1 | Song Jiang |
| 2 | Zhang Sanfeng |
| 3 | Miejue Shitai |
| 4 | Lin Chaoying |
+-------+---------------+
29 rows in set (0.00 sec)
将一张表与另一张表进行“union”多表纵向合并：
MariaDB [hellodb]> insert teachers (name,age,gender)values('abc',50,'F');
Query OK, 1 row affected (0.00 sec)

MariaDB [hellodb]> select * from teachers;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
| 5 | abc | 50 | F |
+-----+---------------+-----+--------+
5 rows in set (0.00 sec)

MariaDB [hellodb]> select * from teacher2;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
+-----+---------------+-----+--------+
2 rows in set (0.00 sec)

MariaDB [hellodb]> select from teachers union select from teacher2;
+-----+---------------+-----+--------+
| TID | Name | Age | Gender |
+-----+---------------+-----+--------+
| 1 | Song Jiang | 45 | M |
| 2 | Zhang Sanfeng | 94 | M |
| 3 | Miejue Shitai | 77 | F |
| 4 | Lin Chaoying | 93 | F |
| 5 | abc | 50 | F |
+-----+---------------+-----+--------+
5 rows in set (0.00 sec)
“inner join ”取二者交集：
MariaDB [hellodb]> select from students inner join teachers on students.teacherid=teachers.tid;
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
| StuID | Name | Age | Gender | ClassID | TeacherID | TID | Name | Age | Gender |
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
| 5 | Yu Yutong | 26 | M | 3 | 1 | 1 | Song Jiang | 45 | M |
| 25 | Sun Dasheng | 100 | M | NULL | 1 | 1 | Song Jiang | 45 | M |
| 1 | Shi Zhongyu | 22 | M | 2 | 3 | 3 | Miejue Shitai | 77 | F |
| 4 | Ding Dian | 32 | M | 4 | 4 | 4 | Lin Chaoying | 93 | F |
+-------+-------------+-----+--------+---------+-----------+-----+---------------+-----+--------+
4 rows in set (0.00 sec)
设置别名 as ，并横向合并：
MariaDB [hellodb]> select stuid,s.name,s.age,tid,t.name,t.age from students as s inner join teachers t on s.teacherid=t.tid;
+-------+-------------+-----+-----+---------------+-----+
| stuid | name | age | tid | name | age |
+-------+-------------+-----+-----+---------------+-----+
| 5 | Yu Yutong | 26 | 1 | Song Jiang | 45 |
| 25 | Sun Dasheng | 100 | 1 | Song Jiang | 45 |
| 1 | Shi Zhongyu | 22 | 3 | Miejue Shitai | 77 |
| 4 | Ding Dian | 32 | 4 | Lin Chaoying | 93 |
+-------+-------------+-----+-----+---------------+-----+
4 rows in set (0.00 sec)
为了看得清楚，再次设置别名，使其名字，年龄都显示更清楚：
MariaDB [hellodb]> select stuid,s.name studentname,s.age studentage,tid,t.name teachername,t.age teacherage from students as s inner join teachers t on s.teacherid=t.tid;
+-------+-------------+------------+-----+---------------+------------+
| stuid | studentname | studentage | tid | teachername | teacherage |
+-------+-------------+------------+-----+---------------+------------+
| 5 | Yu Yutong | 26 | 1 | Song Jiang | 45 |
| 25 | Sun Dasheng | 100 | 1 | Song Jiang | 45 |
| 1 | Shi Zhongyu | 22 | 3 | Miejue Shitai | 77 |
| 4 | Ding Dian | 32 | 4 | Lin Chaoying | 93 |
+-------+-------------+------------+-----+---------------+------------+
4 rows in set (0.00 sec)
“ left join ”左外连接：
MariaDB [hellodb]> select from students left join teachers on students.teacherid=teachers.tid;
+-------+---------------+-----+--------+---------+-----------+------+---------------+------+--------+
| StuID | Name | Age | Gender | ClassID | TeacherID | TID | Name | Age | Gender |
+-------+---------------+-----+--------+---------+-----------+------+---------------+------+--------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 | 3 | Miejue Shitai | 77 | F |
| 2 | Shi Potian | 22 | M | 1 | 7 | NULL | NULL | NULL | NULL |
| 3 | Xie Yanke | 53 | M | 2 | 16 | NULL | NULL | NULL | NULL |
| 4 | Ding Dian | 32 | M | 4 | 4 | 4 | Lin Chaoying | 93 | F |
| 5 | Yu Yutong | 26 | M | 3 | 1 | 1 | Song Jiang | 45 | M |
| 6 | Shi Qing | 46 | M | 5 | NULL | NULL | NULL | NULL | NULL |
| 7 | Xi Ren | 19 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL | NULL | NULL | NULL | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL | NULL | NULL | NULL | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL | NULL | NULL | NULL | NULL |
| 13 | Tian Boguang | 33 | M | 2 | NULL | NULL | NULL | NULL | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL | NULL | NULL | NULL | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL | NULL | NULL | NULL | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL | NULL | NULL | NULL | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL | NULL | NULL | NULL | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL | NULL | NULL | NULL | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL | NULL | NULL | NULL | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL | NULL | NULL | NULL | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL | NULL | NULL | NULL | NULL |
| 25 | Sun Dasheng | 100 | M | NULL | 1 | 1 | Song Jiang | 45 | M |
+-------+---------------+-----+--------+---------+-----------+------+---------------+------+--------+
25 rows in set (0.00 sec) 左边全要，右边只要有交集的部分!
学生和教师的交集，设置别名了的左外连接：
MariaDB [hellodb]> select stuid, s.name studentname,tid,t.name teachername from students as s left join teachers as t on s.teacherid=t.tid;
+-------+---------------+------+---------------+
| stuid | studentname | tid | teachername |
+-------+---------------+------+---------------+
| 1 | Shi Zhongyu | 3 | Miejue Shitai |
| 2 | Shi Potian | NULL | NULL |
| 3 | Xie Yanke | NULL | NULL |
| 4 | Ding Dian | 4 | Lin Chaoying |
| 5 | Yu Yutong | 1 | Song Jiang |
| 6 | Shi Qing | NULL | NULL |
| 7 | Xi Ren | NULL | NULL |
| 8 | Lin Daiyu | NULL | NULL |
| 9 | Ren Yingying | NULL | NULL |
| 10 | Yue Lingshan | NULL | NULL |
| 11 | Yuan Chengzhi | NULL | NULL |
| 12 | Wen Qingqing | NULL | NULL |
| 13 | Tian Boguang | NULL | NULL |
| 14 | Lu Wushuang | NULL | NULL |
| 15 | Duan Yu | NULL | NULL |
| 16 | Xu Zhu | NULL | NULL |
| 17 | Lin Chong | NULL | NULL |
| 18 | Hua Rong | NULL | NULL |
| 19 | Xue Baochai | NULL | NULL |
| 20 | Diao Chan | NULL | NULL |
| 21 | Huang Yueying | NULL | NULL |
| 22 | Xiao Qiao | NULL | NULL |
| 23 | Ma Chao | NULL | NULL |
| 24 | Xu Xian | NULL | NULL |
| 25 | Sun Dasheng | 1 | Song Jiang |
+-------+---------------+------+---------------+
25 rows in set (0.00 sec)
“ right ”右外连接：
MariaDB [hellodb]> select stuid, s.name studentname,tid,t.name teachername from students as s right join teachers as t on s.teacherid=t.tid;
+-------+-------------+-----+---------------+
| stuid | studentname | tid | teachername |
+-------+-------------+-----+---------------+
| 1 | Shi Zhongyu | 3 | Miejue Shitai |
| 4 | Ding Dian | 4 | Lin Chaoying |
| 5 | Yu Yutong | 1 | Song Jiang |
| 25 | Sun Dasheng | 1 | Song Jiang |
| NULL | NULL | 2 | Zhang Sanfeng |
| NULL | NULL | 5 | abc |
+-------+-------------+-----+---------------+
6 rows in set (0.00 sec)
实现左外连接只取没有交集的部分：
MariaDB [hellodb]> select stuid, s.name teacherid,tid,t.name teachername from students as s left join teachers as t on s.teacherid=t.tid where t.tid is null;
+-------+---------------+------+-------------+
| stuid | teacherid | tid | teachername |
+-------+---------------+------+-------------+
| 1 | Shi Zhongyu | NULL | NULL |
| 2 | Shi Potian | NULL | NULL |
| 3 | Xie Yanke | NULL | NULL |
| 4 | Ding Dian | NULL | NULL |
| 5 | Yu Yutong | NULL | NULL |
| 6 | Shi Qing | NULL | NULL |
| 7 | Xi Ren | NULL | NULL |
| 8 | Lin Daiyu | NULL | NULL |
| 9 | Ren Yingying | NULL | NULL |
| 10 | Yue Lingshan | NULL | NULL |
| 11 | Yuan Chengzhi | NULL | NULL |
| 12 | Wen Qingqing | NULL | NULL |
| 13 | Tian Boguang | NULL | NULL |
| 14 | Lu Wushuang | NULL | NULL |
| 15 | Duan Yu | NULL | NULL |
| 16 | Xu Zhu | NULL | NULL |
| 17 | Lin Chong | NULL | NULL |
| 18 | Hua Rong | NULL | NULL |
| 19 | Xue Baochai | NULL | NULL |
| 20 | Diao Chan | NULL | NULL |
| 21 | Huang Yueying | NULL | NULL |
| 22 | Xiao Qiao | NULL | NULL |
| 23 | Ma Chao | NULL | NULL |
| 24 | Xu Xian | NULL | NULL |
| 25 | Sun Dasheng | NULL | NULL |
+-------+---------------+------+-------------+
25 rows in set (0.00 sec)
完全外连接：（先挑出来左外连接，再挑出来右外连接，再union，union有去重的特性，共27条记录）
MariaDB [hellodb]> select stuid, s.name studentname,tid,t.name teachername from students as s left join teachers as t on s.teacherid=t.tid
-> union
-> select stuid, s.name studentname,tid,t.name teachername from students as s right join teachers as t on s.teacherid=t.tid;
+-------+---------------+------+---------------+
| stuid | studentname | tid | teachername |
+-------+---------------+------+---------------+
| 1 | Shi Zhongyu | 3 | Miejue Shitai |
| 2 | Shi Potian | NULL | NULL |
| 3 | Xie Yanke | NULL | NULL |
| 4 | Ding Dian | 4 | Lin Chaoying |
| 5 | Yu Yutong | 1 | Song Jiang |
| 6 | Shi Qing | NULL | NULL |
| 7 | Xi Ren | NULL | NULL |
| 8 | Lin Daiyu | NULL | NULL |
| 9 | Ren Yingying | NULL | NULL |
| 10 | Yue Lingshan | NULL | NULL |
| 11 | Yuan Chengzhi | NULL | NULL |
| 12 | Wen Qingqing | NULL | NULL |
| 13 | Tian Boguang | NULL | NULL |
| 14 | Lu Wushuang | NULL | NULL |
| 15 | Duan Yu | NULL | NULL |
| 16 | Xu Zhu | NULL | NULL |
| 17 | Lin Chong | NULL | NULL |
| 18 | Hua Rong | NULL | NULL |
| 19 | Xue Baochai | NULL | NULL |
| 20 | Diao Chan | NULL | NULL |
| 21 | Huang Yueying | NULL | NULL |
| 22 | Xiao Qiao | NULL | NULL |
| 23 | Ma Chao | NULL | NULL |
| 24 | Xu Xian | NULL | NULL |
| 25 | Sun Dasheng | 1 | Song Jiang |
| NULL | NULL | 2 | Zhang Sanfeng |
| NULL | NULL | 5 | abc |
+-------+---------------+------+---------------+
27 rows in set (0.00 sec)
使用子查询挑出除了交集以外的部分：（子查询：将一个select命令嵌套入另一个select命令中）
MariaDB [hellodb]> select avg(age) from students;
+----------+
| avg(age) |
+----------+
| 27.4000 |
+----------+
1 row in set (0.00 sec)

MariaDB [hellodb]> select from students where age < (select avg(age) from students);
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
| 1 | Shi Zhongyu | 22 | M | 2 | 3 |
| 2 | Shi Potian | 22 | M | 1 | 7 |
| 5 | Yu Yutong | 26 | M | 3 | 1 |
| 7 | Xi Ren | 19 | F | 3 | NULL |
| 8 | Lin Daiyu | 17 | F | 7 | NULL |
| 9 | Ren Yingying | 20 | F | 6 | NULL |
| 10 | Yue Lingshan | 19 | F | 3 | NULL |
| 11 | Yuan Chengzhi | 23 | M | 6 | NULL |
| 12 | Wen Qingqing | 19 | F | 1 | NULL |
| 14 | Lu Wushuang | 17 | F | 3 | NULL |
| 15 | Duan Yu | 19 | M | 4 | NULL |
| 16 | Xu Zhu | 21 | M | 1 | NULL |
| 17 | Lin Chong | 25 | M | 4 | NULL |
| 18 | Hua Rong | 23 | M | 7 | NULL |
| 19 | Xue Baochai | 18 | F | 6 | NULL |
| 20 | Diao Chan | 19 | F | 7 | NULL |
| 21 | Huang Yueying | 22 | F | 6 | NULL |
| 22 | Xiao Qiao | 20 | F | 1 | NULL |
| 23 | Ma Chao | 23 | M | 4 | NULL |
| 24 | Xu Xian | 27 | M | NULL | NULL |
+-------+---------------+-----+--------+---------+-----------+
20 rows in set (0.01 sec)
上述内容是查询小于平均年龄小于27的部分!
删除交集部分：（取出：你有的，我没有的，我有的，你没有的）
MariaDB [hellodb]> select from (select s.stuid,s.name s_name,s.teacherid,t.tid,t.name t_name from students s left join teachers t on s.teacherid=t.tid union select s.stuid,s.name s_name,s.teacherid,t.tid,t.name t_name from students s right join teachers t on s.teacherid=t.tid) as a where a.teacherid is null or a.tid is null;
+-------+---------------+-----------+------+---------------+
| stuid | s_name | teacherid | tid | t_name |
+-------+---------------+-----------+------+---------------+
| 2 | Shi Potian | 7 | NULL | NULL |
| 3 | Xie Yanke | 16 | NULL | NULL |
| 6 | Shi Qing | NULL | NULL | NULL |
| 7 | Xi Ren | NULL | NULL | NULL |
| 8 | Lin Daiyu | NULL | NULL | NULL |
| 9 | Ren Yingying | NULL | NULL | NULL |
| 10 | Yue Lingshan | NULL | NULL | NULL |
| 11 | Yuan Chengzhi | NULL | NULL | NULL |
| 12 | Wen Qingqing | NULL | NULL | NULL |
| 13 | Tian Boguang | NULL | NULL | NULL |
| 14 | Lu Wushuang | NULL | NULL | NULL |
| 15 | Duan Yu | NULL | NULL | NULL |
| 16 | Xu Zhu | NULL | NULL | NULL |
| 17 | Lin Chong | NULL | NULL | NULL |
| 18 | Hua Rong | NULL | NULL | NULL |
| 19 | Xue Baochai | NULL | NULL | NULL |
| 20 | Diao Chan | NULL | NULL | NULL |
| 21 | Huang Yueying | NULL | NULL | NULL |
| 22 | Xiao Qiao | NULL | NULL | NULL |
| 23 | Ma Chao | NULL | NULL | NULL |
| 24 | Xu Xian | NULL | NULL | NULL |
| NULL | NULL | NULL | 2 | Zhang Sanfeng |
| NULL | NULL | NULL | 5 | abc |
+-------+---------------+-----------+------+---------------+
23 rows in set (0.00 sec)

原文地址：http://blog.51cto.com/14128387/2350956

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