mysql基础
1. 关系型数据库介绍
1.1 数据结构模型
数据结构模型主要有：
层次模型
网状结构
关系模型
关系模型：
二维关系：row，column
数据库管理系统：DBMS
关系：Relational，RDBMS
1.2 RDBMS专业名词
常见的关系型数据库管理系统：
MySQL：MySQL，MariaDB，Percona-Server
PostgreSQL：简称为pgsql
Oracle
MSSQL
SQL：Structure Query Language，结构化查询语言
约束：constraint，向数据表提供的数据要遵守的限制
主键约束：一个或多个字段的组合，填入的数据必须能在本表中唯一标识本行。且必须提供数据，不能为空（NOT NULL）。
一个表只能存在一个
惟一键约束：一个或多个字段的组合，填入的数据必须能在本表中唯一标识本行。允许为空（NULL）
一个表可以存在多个
外键约束：一个表中的某字段可填入数据取决于另一个表的主键已有的数据
检查性约束
索引：将表中的一个或多个字段中的数据复制一份另存，并且这些数据需要按特定次序排序存储
1.3 关系型数据库的常见组件
关系型数据库的常见组件有：
数据库：database
表：table，由行（row）和列（column）组成
索引：index
视图：view
用户：user
权限：privilege
存储过程：procedure
存储函数：function
触发器：trigger
事件调度器：event scheduler
1.4 SQL语句
SQL语句有三种类型：
DDL：Data Defination Language，数据定义语言
DML：Data Manipulation Language，数据操纵语言
DCL：Data Control Language，数据控制语言
| SQL语句类型 | 对应操作 |
| DDL |
CREATE：创建, DROP：删除, ALTER：修改 |
| DML |
INSERT：向表中插入数据, DELETE：删除表中数据, UPDATE：更新表中数据, SELECT：查询表中数据 |
| DCL | GRANT：授权,REVOKE：移除授权 |
DDL操作
数据库创建语法：create database []if not exists] 'db_name' MariaDB [(none)]> create database if not exists test; Query OK, 1 row affected (0.001 sec) MariaDB [(none)]> show databases; +--------------------+ | Database | +--------------------+ | information_schema | | mysql | | performance_schema | | school | | test | +--------------------+ 5 rows in set (0.056 sec) 删除数据库语法：drop database [if exists] ‘db_name’ MariaDB [(none)]> drop database if exists test; Query OK, 0 rows affected (0.055 sec) MariaDB [(none)]> show databases; +--------------------+ | Database | +--------------------+ | information_schema | | mysql | | performance_schema | | school | +--------------------+ 4 rows in set (0.000 sec)
DCL-创建授权grant
权限类型(priv_type)
| 权限类型 | 代表什么？ |
| ALL | 所有权限 |
| SELECT | 读取内容的权限 |
| INSERT | 插入内容的权限 |
| UPDATE | 更新内容的权限 |
| DELETE | 删除内容的权限 |
指定要操作的对象db_name.table_name
| 表示方式 | 意义 |
| *.* | 所有库的所有表 |
| db_name | 指定库的所有表 |
| DELETE | 删除内容的权限 |
mysql安装与配置：
1.安装mariadb的所有服务，设置开机自动启动并查看端口号。
语法：
//语法：mysql [OPTIONS] [database]
//常用的OPTIONS： -uUSERNAME
//指定用户名，默认为root -hHOST
//指定服务器主机，默认为localhost，推荐使用ip地址 -pPASSWORD
//指定用户的密码 -P#
//指定数据库监听的端口，这里的#需用实际的端口号代替，如-P3307 -V
//查看当前使用的mysql版本 -e
//不登录mysql执行sql语句后退出，常用于脚本
[root@NaNaQi ~]# mount /dev/cdrom /mnt/
mount: /mnt: WARNING: device write-protected, mounted read-only.
[root@NaNaQi ~]#
[root@NaNaQi ~]# yum -y install mariadb*
[root@NaNaQi ~]# systemctl enable --now mariadb
Created symlink /etc/systemd/system/mysql.service → /usr/lib/systemd/system/mariadb.service.
Created symlink /etc/systemd/system/mysqld.service → /usr/lib/systemd/system/mariadb.service.
Created symlink /etc/systemd/system/multi-user.target.wants/mariadb.service → /usr/lib/systemd/system/mariadb.service.
[root@NaNaQi ~]# ss -antl
State Recv-Q Send-Q Local Address:Port Peer Address:Port
LISTEN 0 128 0.0.0.0:22 0.0.0.0:*
LISTEN 0 80 0.0.0.0:3306 0.0.0.0:*
LISTEN 0 128 [::]:22 [::]:*
2.给数据库的root管理员账户设置密码123，再修改成1234
[root@NaNaQi ~]# mysql -uroot Welcome to the MariaDB monitor. Commands end with ; or \g. Your MariaDB connection id is 10 Server version: 10.3.11-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)]> set password = password('123'); #这里的password是加密 Query OK, 0 rows affected (0.002 sec) MariaDB [(none)]> Ctrl-C -- exit! Aborted 验证 [root@NaNaQi ~]# mysql -uroot -p Enter password: Welcome to the MariaDB monitor. Commands end with ; or \g. Your MariaDB connection id is 11 Server version: 10.3.11-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)]> 修改密码 [root@NaNaQi ~]# mysql -uroot -p Enter password: Welcome to the MariaDB monitor. Commands end with ; or \g. Your MariaDB connection id is 12 Server version: 10.3.11-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)]> alter user root@localhost identified by '1234'; Query OK, 0 rows affected (0.001 sec) MariaDB [(none)]> quit Bye
3.安全初始化数据库，更改root密码为12345
[root@NaNaQi ~]# mysql_secure_installation NOTE: RUNNING ALL PARTS OF THIS SCRIPT IS RECOMMENDED FOR ALL MariaDB SERVERS IN PRODUCTION USE! PLEASE READ EACH STEP CAREFULLY! In order to log into MariaDB to secure it, we'll need the current password for the root user. If you've just installed MariaDB, and you haven't set the root password yet, the password will be blank, so you should just press enter here. Enter current password for root (enter for none): OK, successfully used password, moving on... Setting the root password ensures that nobody can log into the MariaDB root user without the proper authorisation. You already have a root password set, so you can safely answer 'n'. Change the root password? [Y/n] y #修改root密码? New password: Re-enter new password: Password updated successfully! Reloading privilege tables.. ... Success! By default, a MariaDB installation has an anonymous user, allowing anyone to log into MariaDB without having to have a user account created for them. This is intended only for testing, and to make the installation go a bit smoother. You should remove them before moving into a production environment. Remove anonymous users? [Y/n] y #删除匿名用户? ... Success! Normally, root should only be allowed to connect from 'localhost'. This ensures that someone cannot guess at the root password from the network. Disallow root login remotely? [Y/n] n #禁止root远程登录? ... skipping. By default, MariaDB comes with a database named 'test' that anyone can access. This is also intended only for testing, and should be removed before moving into a production environment. Remove test database and access to it? [Y/n] y #删除测试数据库并访问它? - Dropping test database... ... Success! - Removing privileges on test database... ... Success! Reloading the privilege tables will ensure that all changes made so far will take effect immediately. Reload privilege tables now? [Y/n] y #现在重新加载特权表吗? ... Success! Cleaning up... All done! If you've completed all of the above steps, your MariaDB installation should now be secure. Thanks for using MariaDB!
4.用两种方法查看数据库
MariaDB [(none)]> show databases; +--------------------+ | Database | +--------------------+ | information_schema | | mysql | | performance_schema | +--------------------+ 3 rows in set (0.000 sec) MariaDB [(none)]> quit Bye [root@NaNaQi ~]# mysql -uroot -p12345 -e 'show databases;' +--------------------+ | Database | +--------------------+ | information_schema | | mysql | | performance_schema | +--------------------+
5.创建数据库school之后再删除
MariaDB [(none)]> create database if not exists school; Query OK, 0 rows affected, 1 warning (0.000 sec) MariaDB [(none)]> show databases; +--------------------+ | Database | +--------------------+ | information_schema | | mysql | | performance_schema | | school | +--------------------+ 4 rows in set (0.000 sec) MariaDB [(none)]> drop database if exists school; Query OK, 0 rows affected (0.001 sec) MariaDB [(none)]> show databases; +--------------------+ | Database | +--------------------+ | information_schema | | mysql | | performance_schema | +--------------------+ 3 rows in set (0.001 sec)
6.创建数据库school，在其里面创建表student，表里面有id，name，age，使用desc命令查看表 student数据。
MariaDB [(none)]> create database school; Query OK, 1 row affected (0.000 sec) MariaDB [(none)]> use school; Database changed MariaDB [school]> show tables; Empty set (0.000 sec) MariaDB [school]> create table student(id int not null,name varchar(50)null,age tinyint); Query OK, 0 rows affected (0.002 sec) MariaDB [school]> show tables; +------------------+ | Tables_in_school | +------------------+ | student | +------------------+ 1 row in set (0.000 sec) MariaDB [school]> desc student; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | NO | | NULL | | | name | varchar(50) | YES | | NULL | | | age | tinyint(4) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 3 rows in set (0.001 sec)
7.修改表student，插入班级class，之后再删除age一行，最后删除表student。
MariaDB [school]> alter table student add class varchar(20); Query OK, 0 rows affected (0.002 sec) Records: 0 Duplicates: 0 Warnings: 0 MariaDB [school]> desc student; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | NO | | NULL | | | name | varchar(50) | YES | | NULL | | | age | tinyint(4) | YES | | NULL | | | class | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 4 rows in set (0.001 sec) MariaDB [school]> alter table student drop age; Query OK, 0 rows affected (0.005 sec) Records: 0 Duplicates: 0 Warnings: 0 MariaDB [school]> desc student; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | NO | | NULL | | | name | varchar(50) | YES | | NULL | | | class | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 3 rows in set (0.001 sec) MariaDB [school]> drop table student; Query OK, 0 rows affected (0.038 sec) MariaDB [school]> show tables; Empty set (0.000 sec)
8.授权主机，与192.168.44.129登录数据库的所有数据库与所有表，权限为所有权限，用户为root，密码为12345
MariaDB [school]> grant all on *.* to 'root'@'192.168.27.149' identified by '12345';Query OK, 0 rows affected (0.002 sec) MariaDB [school]> grant all on *.* to 'root'@'192.168.44.129' identified by '12345'; Query OK, 0 rows affected (0.001 sec)
9.查看root用户在的192.168.27.149的权限，并取消root用户的192.168.44.129的授权
MariaDB [school]> show grants for 'root'@'192.168.44.129'; +------------------------------------------------------------------------------------------------- | Grants for root@192.168.44.129 +------------------------------------------------------------------------------------------------ | GRANT SELECT, INSERT, UPDATE, CREATE, DROP, RELOAD, SHUTDOWN, PROCESS, FILE, REFERENCES, INDEX, ALTER, SHOW DATABASES, SUPER, CREATE TEMPORARY TABLES, LOCK TABLES, EXECUTE, REPLICATION SLAVE, REPLICATION CLIENT, CREATE VIEW, SHOW VIEW, CREATE ROUTINE, ALTER ROUTINE, CREATE USER, EVENT, TRIGGER, CREATE TABLESPACE, DELETE VERSIONING ROWS ON *.* TO 'root'@'192.168.44.129' IDENTIFIED BY PASSWORD '*00A51F3F48415C7D4E8908980D443C29C69B60C9' | +------------------------------------------------------------------------------------------------- 1 row in set (0.000 sec)
10.在school数据库中创建表student，在其中先单次插入一条数据，再连续插入五条数据。表student包含id,name,age。
MariaDB [school]> use school; Database changed MariaDB [school]> create table student (id int not null,name varchar(50),age tinyint); Query OK, 0 rows affected (0.039 sec) MariaDB [school]> show tables; +------------------+ | Tables_in_school | +------------------+ | student | +------------------+ 1 row in set (0.001 sec) MariaDB [school]> desc student; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | NO | | NULL | | | name | varchar(50) | YES | | NULL | | | age | tinyint(4) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 3 rows in set (0.001 sec) MariaDB [school]> insert into student values(1,'tom',15); Query OK, 1 row affected (0.001 sec) MariaDB [school]> select * from student; +----+------+------+ | id | name | age | +----+------+------+ | 1 | tom | 15 | +----+------+------+ 1 row in set (0.001 sec) MariaDB [school]> insert into student values(2,'zhangshan',20),(3,'lisi',18),(4,'wangwu',20),(5,'zhaosan',13),(6,'qianliu',14); Query OK, 5 rows affected (0.001 sec) Records: 5 Duplicates: 0 Warnings: 0 MariaDB [school]> select * from student; +----+-----------+------+ | id | name | age | +----+-----------+------+ | 1 | tom | 15 | | 2 | zhangshan | 20 | | 3 | lisi | 18 | | 4 | wangwu | 20 | | 5 | zhaosan | 13 | | 6 | qianliu | 14 | +----+-----------+------+ 6 rows in set (0.000 sec)
11.创建表teacher，设置id为主键且自动增长。（只有主键才能设置自动增长）
MariaDB [school]> create table teacher(id int not null primary key auto_increment,name varchar(20) not null,age tinyint,salary float); Query OK, 0 rows affected (0.004 sec) MariaDB [school]> desc teacher; +--------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(20) | NO | | NULL | | | age | tinyint(4) | YES | | NULL | | | salary | float | YES | | NULL | | +--------+-------------+------+-----+---------+----------------+ 4 rows in set (0.001 sec)
12.往表teacher里插入三条数据，再往表teacher里连续插入多条数据。（需指定插入数据的字段）
MariaDB [school]> insert into teacher values(1,'chengsongling',21,4000); Query OK, 1 row affected (0.001 sec) MariaDB [school]> insert into teacher values(2,'chengsongling',21,4000); Query OK, 1 row affected (0.001 sec) MariaDB [school]> insert into teacher values(3,'taochi',21,4000); Query OK, 1 row affected (0.001 sec) MariaDB [school]> select * from teacher; +----+---------------+------+--------+ | id | name | age | salary | +----+---------------+------+--------+ | 1 | chengsongling | 21 | 4000 | | 2 | chengsongling | 21 | 4000 | | 3 | taochi | 21 | 4000 | +----+---------------+------+--------+ 3 rows in set (0.000 sec) MariaDB [school]> insert into teacher(name,age,salary) values('meijianbiao',30,9000),('mufeng',23,5000),('fangxinxin',24,10000),('leichen',21,10000),('yuqinhao',50,100000),('tanghaolun',10,10000); Query OK, 6 rows affected (0.001 sec) Records: 6 Duplicates: 0 Warnings: 0 MariaDB [school]> select * from teacher; +----+---------------+------+--------+ | id | name | age | salary | +----+---------------+------+--------+ | 1 | chengsongling | 21 | 4000 | | 2 | chengsongling | 21 | 4000 | | 3 | taochi | 21 | 4000 | | 4 | meijianbiao | 30 | 9000 | | 5 | mufeng | 23 | 5000 | | 6 | fangxinxin | 24 | 10000 | | 7 | leichen | 21 | 10000 | | 8 | yuqinhao | 50 | 100000 | | 9 | tanghaolun | 10 | 10000 | +----+---------------+------+--------+ 9 rows in set (0.000 sec)
13.修改表teacher中的数据,先把id为9的那行数据年龄改成35，再把年龄改成30，工资改成50000。
MariaDB [school]> update teacher set age = 35 where id = 9; Query OK, 1 row affected (0.001 sec) Rows matched: 1 Changed: 1 Warnings: 0 MariaDB [school]> select * from teacher; +----+---------------+------+--------+ | id | name | age | salary | +----+---------------+------+--------+ | 1 | chengsongling | 21 | 4000 | | 2 | chengsongling | 21 | 4000 | | 3 | taochi | 21 | 4000 | | 4 | meijianbiao | 30 | 9000 | | 5 | mufeng | 23 | 5000 | | 6 | fangxinxin | 24 | 10000 | | 7 | leichen | 21 | 10000 | | 8 | yuqinhao | 50 | 100000 | | 9 | tanghaolun | 35 | 10000 | +----+---------------+------+--------+ 9 rows in set (0.001 sec) ariaDB [school]> update teacher set age = 30,salary = 50000 where id = 9; Query OK, 1 row affected (0.003 sec) Rows matched: 1 Changed: 1 Warnings: 0 MariaDB [school]> select * from teacher; +----+---------------+------+--------+ | id | name | age | salary | +----+---------------+------+--------+ | 1 | chengsongling | 21 | 4000 | | 2 | chengsongling | 21 | 4000 | | 3 | taochi | 21 | 4000 | | 4 | meijianbiao | 30 | 9000 | | 5 | mufeng | 23 | 5000 | | 6 | fangxinxin | 24 | 10000 | | 7 | leichen | 21 | 10000 | | 8 | yuqinhao | 50 | 100000 | | 9 | tanghaolun | 30 | 50000 | +----+---------------+------+--------+ 9 rows in set (0.001 sec)
14.查找teacher表name与salary一项,再给name设置显示的别名为姓名，给salary设置显示的别名为薪资。
MariaDB [school]> select name,salary from teacher; +---------------+--------+ | name | salary | +---------------+--------+ | chengsongling | 4000 | | chengsongling | 4000 | | taochi | 4000 | | meijianbiao | 9000 | | mufeng | 5000 | | fangxinxin | 10000 | | leichen | 10000 | | yuqinhao | 100000 | | tanghaolun | 50000 | +---------------+--------+ 9 rows in set (0.001 sec) MariaDB [school]> select name as '姓名',salary as '薪资' from teacher; +---------------+--------+ | 姓名 | 薪资 | +---------------+--------+ | chengsongling | 4000 | | chengsongling | 4000 | | taochi | 4000 | | meijianbiao | 9000 | | mufeng | 5000 | | fangxinxin | 10000 | | leichen | 10000 | | yuqinhao | 100000 | | tanghaolun | 50000 | +---------------+--------+ 9 rows in set (0.000 sec)
15.先查找薪资为50000的数据的姓名，再查找薪资大于8000的所有记录，最后查找年龄再25到30之间的记录。
MariaDB [school]> select name from teacher where salary = 50000; +------------+ | name | +------------+ | tanghaolun | +------------+ 1 row in set (0.000 sec) MariaDB [school]> select * from teacher where salary > 8000; +----+-------------+------+--------+ | id | name | age | salary | +----+-------------+------+--------+ | 4 | meijianbiao | 30 | 9000 | | 6 | fangxinxin | 24 | 10000 | | 7 | leichen | 21 | 10000 | | 8 | yuqinhao | 50 | 100000 | | 9 | tanghaolun | 30 | 50000 | +----+-------------+------+--------+ 5 rows in set (0.001 sec) MariaDB [school]> select * from teacher where age between 25 and 30; +----+-------------+------+--------+ | id | name | age | salary | +----+-------------+------+--------+ | 4 | meijianbiao | 30 | 9000 | | 9 | tanghaolun | 30 | 50000 | +----+-------------+------+--------+ 2 rows in set (0.001 sec)
16.查找表teacher中以c开头的姓名的记录，再查找以ing结尾的姓名的记录，最后再查找以ing结尾或ao结尾的姓名的记录。
MariaDB [school]> select * from teacher where name like 'c%'; +----+---------------+------+--------+ | id | name | age | salary | +----+---------------+------+--------+ | 1 | chengsongling | 21 | 4000 | | 2 | chengsongling | 21 | 4000 | +----+---------------+------+--------+ 2 rows in set (0.000 sec) MariaDB [school]> select * from teacher where name like '%ing'; +----+---------------+------+--------+ | id | name | age | salary | +----+---------------+------+--------+ | 1 | chengsongling | 21 | 4000 | | 2 | chengsongling | 21 | 4000 | +----+---------------+------+--------+ 2 rows in set (0.000 sec) MariaDB [school]> select * from teacher where name like '%ing' or name like '%ao'; +----+---------------+------+--------+ | id | name | age | salary | +----+---------------+------+--------+ | 1 | chengsongling | 21 | 4000 | | 2 | chengsongling | 21 | 4000 | | 4 | meijianbiao | 30 | 9000 | | 8 | yuqinhao | 50 | 100000 | +----+---------------+------+--------+ 4 rows in set (0.000 sec)
17.往表teacher里插入两条数据，再增加一行字段为department，默认为空，再把id为11的数据的department改为空格。
MariaDB [school]> insert teacher(name,age,salary) values('yanchuang',80,null),('chenben',69,0); Query OK, 2 rows affected (0.001 sec) Records: 2 Duplicates: 0 Warnings: 0 MariaDB [school]> select * from teacher; +----+---------------+------+--------+ | id | name | age | salary | +----+---------------+------+--------+ | 1 | chengsongling | 21 | 4000 | | 2 | chengsongling | 21 | 4000 | | 3 | taochi | 21 | 4000 | | 4 | meijianbiao | 30 | 9000 | | 5 | mufeng | 23 | 5000 | | 6 | fangxinxin | 24 | 10000 | | 7 | leichen | 21 | 10000 | | 8 | yuqinhao | 50 | 100000 | | 9 | tanghaolun | 30 | 50000 | | 10 | yanchuang | 80 | NULL | | 11 | chenben | 69 | 0 | +----+---------------+------+--------+ 11 rows in set (0.000 sec) MariaDB [school]> alter table teacher add department varchar(50) null; Query OK, 0 rows affected (0.001 sec) Records: 0 Duplicates: 0 Warnings: 0 MariaDB [school]> select * from teacher; +----+---------------+------+--------+------------+ | id | name | age | salary | department | +----+---------------+------+--------+------------+ | 1 | chengsongling | 21 | 4000 | NULL | | 2 | chengsongling | 21 | 4000 | NULL | | 3 | taochi | 21 | 4000 | NULL | | 4 | meijianbiao | 30 | 9000 | NULL | | 5 | mufeng | 23 | 5000 | NULL | | 6 | fangxinxin | 24 | 10000 | NULL | | 7 | leichen | 21 | 10000 | NULL | | 8 | yuqinhao | 50 | 100000 | NULL | | 9 | tanghaolun | 30 | 50000 | NULL | | 10 | yanchuang | 80 | NULL | NULL | | 11 | chenben | 69 | 0 | NULL | +----+---------------+------+--------+------------+ 11 rows in set (0.000 sec) MariaDB [school]> update teacher set department = ' ' where id = 11; Query OK, 1 row affected (0.001 sec) Rows matched: 1 Changed: 1 Warnings: 0 MariaDB [school]> select * from teacher; +----+---------------+------+--------+------------+ | id | name | age | salary | department | +----+---------------+------+--------+------------+ | 1 | chengsongling | 21 | 4000 | NULL | | 2 | chengsongling | 21 | 4000 | NULL | | 3 | taochi | 21 | 4000 | NULL | | 4 | meijianbiao | 30 | 9000 | NULL | | 5 | mufeng | 23 | 5000 | NULL | | 6 | fangxinxin | 24 | 10000 | NULL | | 7 | leichen | 21 | 10000 | NULL | | 8 | yuqinhao | 50 | 100000 | NULL | | 9 | tanghaolun | 30 | 50000 | NULL | | 10 | yanchuang | 80 | NULL | NULL | | 11 | chenben | 69 | 0 | | +----+---------------+------+--------+------------+ 11 rows in set (0.000 sec)
18.查找表teacher中dapartment数据不为空的数据。（数据中null等于空，空白不等于空）
MariaDB [school]> select * from teacher where department is not null; +----+---------+------+--------+------------+ | id | name | age | salary | department | +----+---------+------+--------+------------+ | 11 | chenben | 69 | 0 | | +----+---------+------+--------+------------+ 1 row in set (0.000 sec)
19.先在表teacher里按age升序排列，再按salary降序排列。
MariaDB [school]> select * from teacher order by age; +----+---------------+------+--------+------------+ | id | name | age | salary | department | +----+---------------+------+--------+------------+ | 1 | chengsongling | 21 | 4000 | NULL | | 2 | chengsongling | 21 | 4000 | NULL | | 3 | taochi | 21 | 4000 | NULL | | 7 | leichen | 21 | 10000 | NULL | | 5 | mufeng | 23 | 5000 | NULL | | 6 | fangxinxin | 24 | 10000 | NULL | | 4 | meijianbiao | 30 | 9000 | NULL | | 9 | tanghaolun | 30 | 50000 | NULL | | 8 | yuqinhao | 50 | 100000 | NULL | | 11 | chenben | 69 | 0 | | | 10 | yanchuang | 80 | NULL | NULL | +----+---------------+------+--------+------------+ 11 rows in set (0.000 sec) MariaDB [school]> select * from teacher order by salary desc; +----+---------------+------+--------+------------+ | id | name | age | salary | department | +----+---------------+------+--------+------------+ | 8 | yuqinhao | 50 | 100000 | NULL | | 9 | tanghaolun | 30 | 50000 | NULL | | 6 | fangxinxin | 24 | 10000 | NULL | | 7 | leichen | 21 | 10000 | NULL | | 4 | meijianbiao | 30 | 9000 | NULL | | 5 | mufeng | 23 | 5000 | NULL | | 1 | chengsongling | 21 | 4000 | NULL | | 3 | taochi | 21 | 4000 | NULL | | 2 | chengsongling | 21 | 4000 | NULL | | 11 | chenben | 69 | 0 | | | 10 | yanchuang | 80 | NULL | NULL | +----+---------------+------+--------+------------+ 11 rows in set (0.000 sec)
20.先删除表teacher中工资为空的数据，再删除department不为空的数据。最后删除整个表teacher的数据。（使用delete命令删除，表依然会存在，且会保留表结构）
MariaDB [school]> delete from teacher where salary is null; Query OK, 1 row affected (0.050 sec) MariaDB [school]> select * from teacher; +----+---------------+------+--------+------------+ | id | name | age | salary | department | +----+---------------+------+--------+------------+ | 1 | chengsongling | 21 | 4000 | NULL | | 2 | chengsongling | 21 | 4000 | NULL | | 3 | taochi | 21 | 4000 | NULL | | 4 | meijianbiao | 30 | 9000 | NULL | | 5 | mufeng | 23 | 5000 | NULL | | 6 | fangxinxin | 24 | 10000 | NULL | | 7 | leichen | 21 | 10000 | NULL | | 8 | yuqinhao | 50 | 100000 | NULL | | 9 | tanghaolun | 30 | 50000 | NULL | | 11 | chenben | 69 | 0 | | +----+---------------+------+--------+------------+ 10 rows in set (0.035 sec) MariaDB [school]> delete from teacher where department is not null; Query OK, 1 row affected (0.001 sec) MariaDB [school]> select *from teacher; +----+---------------+------+--------+------------+ | id | name | age | salary | department | +----+---------------+------+--------+------------+ | 1 | chengsongling | 21 | 4000 | NULL | | 2 | chengsongling | 21 | 4000 | NULL | | 3 | taochi | 21 | 4000 | NULL | | 4 | meijianbiao | 30 | 9000 | NULL | | 5 | mufeng | 23 | 5000 | NULL | | 6 | fangxinxin | 24 | 10000 | NULL | | 7 | leichen | 21 | 10000 | NULL | | 8 | yuqinhao | 50 | 100000 | NULL | | 9 | tanghaolun | 30 | 50000 | NULL | +----+---------------+------+--------+------------+ 9 rows in set (0.000 sec) MariaDB [school]> delete from teacher; Query OK, 9 rows affected (0.001 sec) MariaDB [school]> select * from teacher; Empty set (0.000 sec) MariaDB [school]> show tables; +------------------+ | Tables_in_school | +------------------+ | student | | teacher | +------------------+ 2 rows in set (0.000 sec)
21.再往teacher表里插入数据，会发现id还在顺延，使用truncate可以彻底删除。之后插入数据会发现id重新开始计数。（使用truncate删除的话数据无法恢复）
MariaDB [school]> insert teacher(name,age,salary) values('tom',20,8000),('jerry',23,6000),('zhangsan',22,9000); Query OK, 3 rows affected (0.001 sec) Records: 3 Duplicates: 0 Warnings: 0 MariaDB [school]> select * from teacher; +----+----------+------+--------+------------+ | id | name | age | salary | department | +----+----------+------+--------+------------+ | 12 | tom | 20 | 8000 | NULL | | 13 | jerry | 23 | 6000 | NULL | | 14 | zhangsan | 22 | 9000 | NULL | +----+----------+------+--------+------------+ 3 rows in set (0.000 sec) MariaDB [school]> truncate teacher; Query OK, 0 rows affected (0.008 sec) MariaDB [school]> select * from teacher; Empty set (0.000 sec) MariaDB [school]> insert teacher(name,age,salary) values('tom',20,8000),('jerry',23,6000),('zhangsan',22,9000); Query OK, 3 rows affected (0.001 sec) Records: 3 Duplicates: 0 Warnings: 0 MariaDB [school]> select * from teacher; +----+----------+------+--------+------------+ | id | name | age | salary | department | +----+----------+------+--------+------------+ | 1 | tom | 20 | 8000 | NULL | | 2 | jerry | 23 | 6000 | NULL | | 3 | zhangsan | 22 | 9000 | NULL | +----+----------+------+--------+------------+ 3 rows in set (0.001 sec)
